abortion See miscarriage.
adaptation Any phenotypic feature that helps an organism survive and reproduce in a particular environment.
adelphophagy Cannibalism by embryos.
afterbirth Remnants of the placenta expelled by a woman after delivery.
aging See senescence.
allantoic sac See allantois.
allantois A saclike membrane present in the embryos of reptiles, birds, and mammals.
allele Any of the possible forms (or classes of forms) of a gene. A diploid individual carries two alleles at each autosomal gene, and these can either be identical in state (in which case the individual is homozygous) or different in state (heterozygous). At each autosomal gene, a population of N diploid individuals harbors 2N alleles, some of whose nucleotide sequences may differ.
allograft An artificial transfer of tissue from one individual to another.
allohormone Any substance that, when transferred directly into the body of a conspecific, induces a physiological response.
alloparental care Care of offspring by individuals other than a parent.
allopatric Inhabiting different geographic areas.
allozyme A genetic form of a protein revealed by protein electrophoresis.
altruism Acting in the interest of others.
amenorrhea A temporary cessation of menstruation.
amnion The innermost embryonic or fetal membrane of an amniotic vertebrate.
amniote Any vertebrate animal that has an amnion (i.e., a reptile, bird, or mammal).
angiosperm A flowering plant with fruit-encased seeds.
anisogamy The bimodal distribution of gamete size (smaller in males, larger in females), which characterizes sexually reproducing organisms.
antagonistic selection Selection pushing on a trait in different directions.
antibody A protein produced by the immune system that reacts to foreign material and thereby protects the body against invasive substances.
antigen A substance that, when introduced into the body, is capable of eliciting an immune response.
aplacental Lacking a placenta.
apomixis Asexual reproduction without meiosis or fertilization.
artificial selection A human-mediated analogue of natural selection in which people promote the differential reproduction of organisms with specific desired traits.
asexual reproduction Any form of reproduction that does not involve a fusion of sex cells.
autism An abnormality beginning in childhood that affects language and communication skills.
autosome A chromosome in the nucleus other than a sex chromosome.
Bateman gradient A statistical regression describing the relationship between mate numbers and progeny production.
bathypelagic Pertaining to the deep sea.
biodiversity Life’s genetic heterogeneity, at any or all levels of biological organization.
birth Parturition.
birth control Any method used either to prevent syngamy or to interrupt a pregnancy at any stage before parturition.
bisexual Composed of male and female individuals.
blastocyst The mammalian embryo at the time of implantation.
blastula An early embryonic stage consisting of a hollow sphere of cells.
bottleneck A severe but temporary reduction in population size.
broadcast spawning The spewing of gametes into an open environment, such as the sea.
brood n, A group of offspring being cared for together by a parent; v, to care for such a group of offspring.
brooding The parental act of caring for a brood.
broodmates Members of a brood.
cannibalism The consumption of conspecific individuals or their body parts.
cartilaginous Composed of cartilage (e.g., skeletons in sharks and rays).
cell A small, membrane-bound unit of life that is usually capable of self-reproduction.
cervix The narrow, lower portion of the uterus, where it joins the inner end of the vagina.
cesarean section A surgical procedure to remove a fetus from a uterus.
chimera An individual composed of a mixture of genetically distinct cells from different zygotes.
chorion The outermost extraembryonic membrane in land vertebrates; in placental mammals the chorion contributes to the formation of the placenta.
chromosome A threadlike structure within a cell that carries genes.
clade A monophyletic group of organisms.
classification (biological) A process of establishing, defining, and ranking biological taxa within hierarchical groups; also, the outcome of this process.
cleidoic Enclosed within a relatively impervious shell.
cloaca The vaginal opening (e.g., of birds and amphibians).
clonal Pertaining to a clone.
clone n, a biological entity (e.g., gene, cell, or multicellular organism) that is genetically identical to another; also, all genetically identical entities that have descended asexually from a given ancestor; v, to produce such genetically identical entities or lineages.
clonemates Two or more organisms that are genetically identical.
clutch The eggs or offspring within a brood.
coadaptation The mutual adaptation of two or more biological entities.
coelum Body cavity.
coevolution The interdependent evolution of two or more interacting species or genomes.
coitus See intercourse.
conception Fertilization; syngamy.
conceptus An early embryo shortly after fertilization.
condom A contraceptive sheath intended to block fertilization.
congeneric Belonging to the same taxonomic genus.
conjoined twins Twins whose bodies are partially joined or fused.
conspecific Belonging to the same taxonomic species.
constitutive Of consistent and essential occurrence. See also facultative.
contraception Any blockage of the fertilization process.
convergent evolution The independent evolution of phenotypic similarities by distantly related or unrelated species.
copulation The physical act of mating.
cryptic female choice Postcopulatory bias in a female’s use of different male gametes to fertilize her eggs.
C-section See cesarean section.
cuckoldry Stolen copulations often resulting in extrapair paternity.
cytonuclear analysis A genetic appraisal based on information from an organism’s cytoplasmic genome (typically mtDNA) and from one or more loci in the nuclear genome.
cytoplasm The portion of a cell outside the nucleus.
dam The female parent of particular offspring.
delayed implantation See embryonic diapause.
delivery Birth.
deoxyribonucleic acid (DNA) The genetic material of most life forms; a double-stranded molecule composed of strings of nucleotides.
dermatophagy Eating of the skin.
diabetes A disease in which the pancreas cannot properly produce or utilize insulin.
diapause Any period of inactivity or suspended growth during ontogeny.
diaphragm (contraceptive) A cuplike device inserted into the vagina to prevent pregnancy.
diploid A usual somatic cell condition wherein two copies of each chromosome are present.
dizygotic twins Twins arising from different zygotes.
dysmenorrhea Menstrual cramps.
ecology The study of the interrelationships among living organisms and their environments.
ectoparasite A parasite that remains on the outside of a host’s body.
ectopic pregnancy Implantation and gestation of an embryo outside the normal uterine cavity.
effective population size The number of individuals in an idealized population displaying the same genetic properties as those observed in an actual population.
egg A female gamete; oocyte. Also, the entire oocyte-containing package laid by a female.
egg mimicry A phenomenon involving the evolution of body parts that resemble eggs.
electrophoresis The movement of charged proteins or nucleic acids through a supporting gel under the influence of an electric current.
embryo An organism in the early stages of development.
embryogenesis The development of an embryo.
embryonic diapause A period of inactivity or suspended growth of an embryo.
endemic Native to, and restricted to, a particular geographic area.
endocrine Pertaining to the body’s production of hormones.
endogenous Produced or naturally occurring within the body.
endometrium The lining of the uterus in mammals.
endoparasite A parasite that resides inside a host’s body for at least part of its life cycle.
endosperm Tissue that surrounds and provides nutrition to the embryo of a flowering plant.
endosymbiosis An intimate symbiotic relationship that begins when one organism takes up perhaps permanent and mutually beneficial residence within another.
enzyme A catalyst (normally a protein) of a specific chemical reaction.
epigamic selection A form of sexual selection that operates via the mating preferences of females for particular males (or sometimes vice versa).
epigenetic Any mechanism during ontogeny that causes phenotypic variation without altering the nucleotide sequences of the genes.
episiotomy A surgical incision on the posterior vaginal wall to facilitate delivery.
estrus A state in which a female mammal is sexually excitable and capable of conceiving.
ethology The study of animal behavior.
eukaryote Any organism in which chromosomes are housed in a membrane-bound nucleus.
evolution Change over time in the genetic composition of a population or species.
evolutionary stable strategy The value at which a trait in a population is immune to permanent invasion by an alternative trait.
exaptation A trait that evolved to serve one function but later shifted to perform another function.
exogenous Produced or naturally occurring outside the body.
expression (of a gene) Activation of a gene to begin the process (RNA formation) that later may eventuate in the production of a protein.
extant Alive today.
extinction The permanent disappearance of a population or species.
extracorporeal fertilization Conception at a site other than the usual within-female location.
facultative Optional; occurring only part of the time. See also constitutive.
fallopian tube See oviduct.
fecundity The number of gametes an individual produces; fertility.
female The sex that produces relatively large gametes.
fertility See fecundity.
fertilization The union of two gametes to produce a zygote; syngamy.
fetus An organism at intermediate stages of development in the uterus (in humans, beginning at about the third month of pregnancy).
filial cannibalism Eating one’s own offspring.
fitness (Darwinian) The contribution of an individual or a genotype to the next generation relative to the contributions of other individuals or genotypes. See also inclusive fitness.
fixation A situation in which an allele has reached 100% frequency in a population.
follicle A pit or hollow (often used of ovarian pockets that release ova).
fraternal twins See dizygotic twins.
frequency-dependent selection A form of natural selection that varies as a function of the frequencies of alternative genotypes in a population.
fry Juvenile fish.
game theory A branch of theoretical population biology that studies how animals optimally act when each individual’s success depends on its own decisions and those of others.
gamete A mature reproductive sex cell (egg or sperm).
gametogenesis The process by which sex cells are produced.
gender The sex of an individual.
gene The basic unit of heredity; usually taken to imply a sequence of nucleotides specifying the production of a polypeptide or other functional product but can also be applied to stretches of DNA with unknown or unspecified function.
gene flow The geographic movement of genes, normally among populations within a species.
gene pool All of the hereditary material in a population or species.
genealogy A record of descent from ancestors.
genetic Pertaining to the study of heredity.
genetic drift Change in allele frequency in a finite population by chance sampling of gametes between generations.
genetic marker See molecular marker.
genome The complete genetic constitution of an organism; can also refer to a particular composite, discrete piece of genetic material, such as mtDNA or cpDNA.
genotype The genetic constitution of an individual with reference to a gene or set of genes.
germ line The lineage of cells leading to an individual’s gametes.
germination The commencement and growth of a propagule or bud.
gestation In viviparous species, the period from conception to birth. In some other species with external parental care, the period from conception to the final departure of independent offspring.
gonad A sex organ. See also ovary, testis.
gonochoristic Pertaining to a sexual system in which each individual is either a male or a female.
gonopodium In fish, an anal fin modified into an intromittent organ.
gravid Carrying eggs or young.
group selection Natural selection acting upon groups of individuals via differences in the traits of those groups.
gymnosperm A plant with naked seeds (i.e., not enclosed in a fruit).
haploid The usual condition of a gametic cell in which one copy of each chromosome is present.
hatch The escape of an embryo from any egg membranes or shell that envelop it.
heredity The phenomenon of familial transmission of genetic material from one generation to the next.
hermaphroditism A condition in which an individual produces both male and female gametes, either sequentially or simultaneously.
heterocannabilism The consumption of conspecific non-kin.
heterochrony An evolutionary change in the onset of development in individuals or their phenotypes.
heterogametic sex The gender that produces gametes each containing one of two different types of sex chromosomes.
heterosis Higher genetic fitness of heterozygotes than homozygotes.
heterospecific Pertaining to another species.
heterozygote A diploid organism possessing two different alleles at a gene.
hibernation The act or condition of being in a dormant or resting state, typically seasonally.
histocompatibility Pertaining to genes that influence the acceptance or rejection of cells or tissues in grafts.
histotrophe Cellular debris and breakdown products of moribund eggs and embryos, sometimes eaten by live gestating embryos.
homeotic gene A gene with profound morphotypic consequences during individual ontogeny.
homicide Murder.
homogametic sex The gender that produces gametes all containing the same type of sex chromosome.
homology Similarity of traits due to inheritance from a shared ancestor.
homospecific Pertaining to the same species.
homozygote A diploid organism possessing two identical alleles at a specified gene.
hormone A chemical substance produced by the body that produces a specific physiological response.
hybridization The successful mating of individuals belonging to genetically different populations or species.
hybridogenesis A quasi-sexual form of reproduction in which egg and sperm unite but germ cells in the resulting offspring later undergo an abnormal meiosis in which gametes carry no paternally derived genes.
hypertension Abnormally high blood pressure.
hypoglycemia Too much glucose in the blood.
identical twins See monozygotic twins.
immunology The science of the immune system.
immunosurveillance Mechanisms by which the immune system monitors foreign substances.
implantation The attachment of an embryo to the uterine wall.
imprinting, genetic A situation in which a gene is expressed in progeny when inherited from one parent but not the other.
inbreeding Mating and reproduction between kin.
inbreeding depression A loss in genetic fitness due to inbreeding.
incestuous Pertaining to matings between close kin.
inclusive fitness An individual’s own genetic fitness as well as his or her effects on the genetic fitness of close relatives.
incubation See gestation, brooding.
infant A recently born offspring.
infanticide The killing of a live-birthed child.
infertility An inability to conceive or reproduce.
inner cell mass Cells that arise mitotically from a zygote and are precursor to the mammalian embryo.
intercourse Mating involving intromission.
intersexual selection See epigamic selection.
intrasexual selection Competition among members of the same sex for mating access.
intromission During mating, entry of a male’s sexual organ into a female.
in vitro fertilization (IVF) Syngamy outside the living body.
invertebrate An animal not possessing a backbone.
isogamy A condition entailing the fusion of gametes of similar size. See also anisogamy.
iteroparity Repeated reproductive cycles within an individual’s lifetime.
kin Genetic relatives.
kin selection A form of natural selection due to individuals favoring the survival and reproduction of genetic relatives.
labor The process of giving birth in viviparous species.
lactation The secretion of milk from mammary glands.
lecithotrophy Embryonic dependence upon yolk for nutrition.
life cycle The sequence of ontogenetic events from zygote to death; one generation.
linked genes Loci carried on the same chromosome.
litter See clutch, brood.
locus (pl. loci) A gene, a location on a chromosome.
madumnal Used in reference to maternally derived alleles present in offspring.
male The sex that produces relatively small gametes.
marsupium An external brood pouch on the abdomen of most female marsupial mammals.
maternal Pertaining to a female parent.
mating system The pattern by which males and females mate (or their gametes unite) in a population. See also monogamy, polygamy, polyandry, polygyny, polygynandry, promiscuity, outcrossing.
matriline A genetic transmission pathway strictly through females.
matrotrophy Direct embryonic dependence upon the mother for nutritional support.
meiosis The cellular process whereby a diploid cell divides to form haploid gametes.
meiotic Pertaining to meiosis.
menarche The time of a girl’s first menstruation.
menopause The cessation of menstruation and the normal termination of fertility as a woman ages.
menses See menstruation.
menstruation The periodic flow of blood and mucosal tissue from the uterus to the outside of a woman’s body via the vagina.
metabolism The sum of all physical and chemical processes by which living matter is produced and maintained, and by which cellular energy is made available to an organism.
metamorphosis A marked structural transformation during an individual’s development.
microbe A very small organism visible only under a microscope.
microsatellite A locus containing tandem repeats of short nucleotide sequences.
miscarriage The natural expulsion and death of an embryo or fetus; in humans, before approximately the 22nd week of gestation. See also abortion.
mitochondrial DNA (mtDNA) A small and typically circular genome housed in the mitochondrion.
mitochondrion (pl. mitochondria) A cytoplasmic organelle that is the site of key metabolic pathways involved in producing cellular energy.
mitosis A process of cell division that produces daughter cells with the same chromosomal constitution as the parent cell.
mitotic Pertaining to mitosis.
molecular clock An evolutionary timepiece based on the evidence that genes or proteins tend to accumulate mutational differences at roughly constant rates in particular lineages.
molecular marker Any nucleic acid or protein used to deduce the genetic relationships or movements of organisms.
monogamy A mating system in which each male and each female has only one mate.
monogenic Attributable to or pertaining to a single genetic locus.
monophyletic Of single evolutionary origin.
monotocous Referring to pregnancies that produce singleton progeny.
monotreme An egg-laying mammal in the Monotremata.
monozygotic twins Twins that trace back to a single fertilized egg.
morphology The visible structures of organisms.
mouthbrooding Parental care of eggs and embryos in the oral cavity.
multicellular Composed of two or more cells.
mutation A change in the genetic constitution of an organism.
natural history The study of nature including organisms and natural phenomena.
natural selection The differential contribution by individuals of different genotypes to the next generation.
nepotism Favoritism directed toward kin.
niche The ecological role of a species in a natural community; an organism’s way of making a living.
norm of reaction The range of phenotypes normally produced by a given genotype.
nucleic acid See deoxyribonucleic acid and ribonucleic acid.
nucleotide A unit of DNA or RNA consisting of a nitrogenous base, a pentose sugar, and a phosphate group.
nucleus (pl. nuclei) A portion of a cell bounded by a membrane and containing chromosomes.
obstetrician A physician who specializes on healthcare during pregnancy and childbirth.
ontogeny The course of development and growth of an individual to maturity.
oocyte (unfertilized) A female gamete, also known as an egg cell, or ovum.
oogenesis The production of oocytes.
oophagy The consumption of eggs.
operational sex ratio The number of males versus females (or their gametes) effectively available for reproduction during the time period under consideration.
organ A part of an animal, such as the heart, that forms a structural and functional unit.
organelle A complex, recognizable structure in the cell cytoplasm (such as a mitochondrion or chloroplast).
outcrossing Mating with another, typically unrelated, individual.
ovary An ovum producing organ.
oviduct A small tube leading from the ovary to the uterus.
ovigerous See gravid.
oviparity Egg-laying.
oviposition The passage of an egg from the mother’s body to the outside.
ovotestis An organ that produces both eggs and sperm.
ovoviviparity A system in which young are delivered alive after having hatched from fertilized eggs inside the mother’s body.
ovulation The release of a mature oocyte from the ovary.
ovule The structure in seed plants that develops into a seed after fertilization of the egg within it.
ovum An unfertilized egg.
padumnal Used in reference to paternally derived alleles present in offspring.
parasite An organism that at some time in its lifecycle is intimately associated with and harms its host.
parasitoid A parasite that feeds for part of its lifecycle within a host’s body and does not have multiple generations per host generation.
parental care Tending of offspring by a parent. See also parental investment.
parental investment Any investment by the parent in an offspring that increases the offspring’s chances of surviving (and hence reproductive success) at the cost of the parent’s ability to invest in other offspring.
parthenogenesis The development of an individual from an egg without fertilization.
partum See parturition.
parturition The exit of an offspring from its parent’s body.
paternal Pertaining to a male parent.
pathogen An organism or microorganism that produces a disease.
pedigree A diagram displaying mating partners and their offspring across generations.
pelagic Pertaining to the open ocean.
periconception The period of time encompassing gametogenesis, syngamy, and early zygotic development.
perineotomy A surgical incision into the perineum.
perineum The portion of the pelvis occupied by the urogenital passages and rectum.
phallodeum A phallus-like extension of the cloaca in some amphibians.
phallus An erect penis.
phenotype Any morphological, physiological, behavioral, or other such characteristics of an organism.
phenotypic plasticity Variation among phenotypes not due directly to genetic differences. See also norm of reaction.
pheromone A chemical message, secreted by an individual, that conveys information to and often elicits a specific response from another individual.
photosynthesis The biochemical process by which a plant uses light to produce carbohydrates from carbon dioxide and water.
phylogenetic Pertaining to phylogeny.
phylogenetic character mapping (PCM) A scientific exercise in which alternative traits are plotted and ancestral states are inferred in a phylogenetic framework.
phylogeny Evolutionary relationships (historical descent) of a group of organisms or species.
phylogeography Study of the spatial distributions of genealogical lineages.
piscine Pertaining to fish.
placenta A disk-shaped organ on the mother’s uterine wall that attaches to the umbilical cord and thereby connects a pregnant dam to her gestating child.
placentation The process of formation of the placenta.
placentotrophy The consumption of the placenta.
plankton Small organisms that float freely in the ocean or other large bodies of water.
planktotrophy A lifecycle that includes a plankton-feeding stage.
planula A flat free-swimming larval stage.
pleiotropy A phenomenon in which a single gene can contribute to more than one phenotype.
poecilogony Any intraspecific polymorphism in larval lifestyles.
pollen A male gamete in plants.
pollen competition Rivalry among pollen grains over fertilization success. See also sperm competition; cryptic female choice.
pollination The transference of pollen to receptive female parts of a plant.
polyandry A mating system in which particular females may have multiple mates but each male typically has only one mate. Also, any situation in which a focal female has two or more mates.
polyembryony The production of genetically identical offspring within a clutch.
polygamy A mating system in which at least some individuals have multiple mates. See also polyandry, polygyny, polygynandry, promiscuity.
polygenic Attributable to or pertaining to multiple genetic loci.
polygynandry A mating system in which both males and females may have several mates each.
polygyny A mating system in which particular males may have multiple mates but each female typically has only one mate. Also, any situation in which a focal male has two or more mates.
polymorphism The presence of two or more distinct forms (traits or genotypes) in a population.
polyphyletic A group of organisms perhaps classified together but tracing to different ancestors.
population All individuals of a species normally inhabiting a defined area.
postzygotic Following the formation of a fertilized egg.
preadaptation See exaptation.
precocious puberty An early onset of puberty.
preeclampsia High blood pressure associated with a pregnancy.
preemie A human baby born prematurely. See also premature birth.
premature birth Early delivery (in humans, before about 37 weeks of gestation).
preterm birth See premature birth.
prezygotic Preceding the formation of a fertilized egg.
procreation Reproduction.
prokaryote Any microorganism that lacks a chromosome-containing, membrane-bound nucleus.
prolonged pregnancy In humans, any gestation that lasts longer than about 41–42 weeks.
promiscuity An extreme form of polygynandry in which each male and female has many mating partners.
protandry A type of hermaphroditism in which an individual is first male and then later in life switches to female.
protein A macromolecule composed of one or more polypeptide chains.
protogyny A type of hermaphroditism in which an individual is first female and then later in life switches to male.
pseudoplacenta A structure somewhat analogous to the placenta in mammals.
puberty The stage of adolescence at which an individual becomes physiologically capable to reproduce.
recombination (genetic) The formation of new combinations of genes, as for example occurs naturally via meiosis and fertilization.
regulatory gene A segment of DNA that exerts operational control over the expression of other genes.
ribonucleic acid (RNA) The genetic material of many viruses, similar in structure to DNA. Also, any of a class of molecules that normally arise in cells from the transcription of DNA.
schizophrenia Any of a group of psychiatric disorders characterized by withdrawal from reality, illogical thinking, and other intellectual disturbances.
secondary sexual traits Phenotypic characters other than the primary reproductive organs that are elaborated in one sex and evolved via sexual selection.
semelparity The occurrence of a single brood during an individual’s lifetime.
senescence A persistent decline with age in the survival probability or reproductive output of an individual due to interior physiological deterioration.
sex allocation The relative parental investment in male versus female reproductive functions.
sex chromosome A chromosome in the cell nucleus involved in distinguishing the two genders.
sex ratio The relative number of males versus females in a population.
sex-role reversal A situation in which sexual selection operates more strongly on females than on males.
sexual dimorphism Consistent differences in secondary sexual traits between males and females.
sexual reproduction Organismal procreation via the generation and fusion of gametes.
sexual selection Selection pressures arising from intraspecific competition for mates.
sexual selection gradient See Bateman gradient.
Siamese twins See conjoined twins.
siblicide One sibling killing another.
siblings Offspring within a brood or clutch.
sire The male parent of particular offspring.
soma See somatic.
somatic Of or pertaining to any cell (or body part) in a multicellular organism other than those destined to become gametes.
species (biological) Groups of actually or potentially interbreeding individuals that are reproductively isolated from other such groups.
sperm A male gamete in animals.
sperm competition Competition among sperm for fertilization success.
spermatheca A female storage organ for sperm.
spermatid One of the four haploid cells produced during each meiosis in males.
spermatogenesis The production of sperm.
spermatophore A packet of sperm.
spermozeugmatum A spherical ball of sperm, as produced by some fishes.
spontaneous abortion See miscarriage.
squamate reptiles Snakes and lizards.
suckle To nurse on a mother’s milk.
superembryonation See superfetation.
superfetation A situation in which two or more cohorts of embryos of different developmental stages co-occur in a pregnancy.
sympatric Inhabiting the same geographic area.
synapomorphy An evolutionarily derived trait shared by two or more related taxa.
syngamy The genetic union of a male gamete and a female gamete.
systematics The comparative study and classification of organisms, particularly with regard to their phylogenetic relationships.
taxon (pl. taxa) A biotic lineage or entity deemed sufficiently distinct from other such lineages as to be worthy of a formal taxonomic name.
taxonomy The practice of naming and classifying organisms.
testis A sperm-producing organ.
tissue A population of cells of the same type performing the same function.
transcription The cellular process by which an RNA molecule is formed from a DNA template.
translation The cellular process by which a polypeptide chain is formed from an RNA template.
trophectoderm Cells that arise mitotically from a zygote, are not part of the embryo itself, but instead are a precursor to the mammalian placenta.
trophic Pertaining to mode of feeding.
trophoblast The nonembryonic part of the blastocyst, later developing into the fetal portion of the placenta.
trophonemata Villous extensions from the uterine wall in some live-bearing fishes.
trophotaeniae Special placenta-like structures found in some live-bearing animals.
umbilical cord A rope-like conduit connecting a pregnant mammal to her gestating child.
ungulate Any large hoof-bearing, grazing mammal.
urethra The canal through which urine is discharged.
uterus A hollow organ in the pelvic region of female mammals wherein the fertilized egg implants and develops during a pregnancy.
vagina The canal leading from the exterior to the uterus in the reproductive tract of females.
variance Statistical variation; the mean squared deviation from the mean.
vas deferens A duct in males that carries sperm from the testis to the urethra.
vasectomy A contraceptive surgical procedure involving removal of all or part of the vas deferens.
vertebrate An animal that possesses a backbone.
vitellogenesis The process of yolk formation in an egg.
viviparity The production and delivery of live offspring from within the body of a parent.
vivipary In botany, the precocious development of sexual progeny directly on a parent plant.
W chromosome In birds, the sex chromosome normally present in females only.
womb A hollow or space where something is generated. See also uterus.
X chromosome The sex chromosome normally present as two copies in female mammals (the homogametic sex), but as only one copy in males (the heterogametic sex).
Y chromosome In mammals, the sex chromosome normally present in males only.
yolk The part of an egg that serves as the primary source of nourishment for an early embryo.
yolk sac An extra-embryonic membrane that surrounds the yolk in reptilian and avian eggs.
Z chromosome The sex chromosome normally present as two copies in male birds (the homogametic sex), but as only one copy in females (the heterogametic sex).
zygote Fertilized egg; the diploid cell arising from the union of male and female haploid gametes.