Discussed in early development section of this chapter.
The three embryonic germ layers are the endoderm, mesoderm, and ectoderm. The thyroid and lung epithelium are of endodermal origin. The testes and the aorta are of mesodermal origin. The nails, lens, and peripheral nerves are of ectodermal origin.
The name blastula refers to an early stage of development in which the embryo consists of a hollow ball of cells surrounding a cavity called the blastocoel.
Cleavage is the rapid division of cells early in the embryo’s development. A determinate cleavage results in cells whose differentiation pathways are clearly defined; these cells are incapable of individually developing into complete organisms. The appearance of the three primary germ layers—the endoderm, mesoderm, and ectoderm—from such processes occurs during gastrulation.
This subtle point about ovulation eludes most people and remains hard to believe until the organs are examined in anatomy class. The ruptured ovarian follicle releases an oocyte into the abdominal cavity, close to the entrance of the fallopian tube. With the aid of beating cilia, the oocyte is drawn into the fallopian tube, through which it travels until it reaches the uterus. If it is fertilized (in the fallopian tube), it will implant in the uterine wall; if it is not fertilized, it will be expelled along with the uterine lining during menstruation.
Sexual reproduction promotes genetic variability through the independent assortment of homologous chromosomes, the crossing over between homologous chromosomes during meiosis, and the random fertilization of an egg by a sperm.
The independent assortment of chromosomes during gametogenesis allows for tremendous genetic variability by creating numerous possible combinations of chromosomes in a given gamete. During metaphase I, homologous chromosomes pair and randomly align at the metaphase plate. The random positioning of the homologous pairs determines which chromosomes are pulled toward each pole of the cell during anaphase. Thus, each resultant daughter cell has a random assortment of chromosomes, some of maternal origin and some of paternal origin.
In addition to independent assortment, a random exchange of genes between chromosomes can occur via recombination. This allows for greater genetic variability by creating new combinations of genes on each chromosome. Recombination occurs during prophase I of meiosis, when homologous pairs of chromosomes align themselves side by side and exchange genetic information in a process called crossing over.
Genetic variability is further enhanced by fertilization. An egg cell containing one of the millions of possible gene combinations fuses with a sperm cell containing another of the millions of possible combinations of different genes to create a zygote with a unique assortment of maternal and paternal genes.
Genetic variability benefits a species because it increases the chances that the offspring will be able to adapt to a myriad of potential environmental stresses and conditions, and promotes evolution over many generations.