THE TERM Embryology, in its widest sense, is applied to the various changes which take place during the growth of an animal from the egg to the adult condition: it is, however, usually restricted to the phenomena which occur before birth.
Embryology may be studied from two aspects: (1) that of ontogeny, which deals only with the development of the individual; and (2) that of phylogeny, which concerns itself with the evolutionary history of the animal kingdom. 1 In vertebrate animals the development of a new being can only take place when a female germ cell or ovum has been fertilized by a male germ cell or spermatozoön.
The ovum is a nucleated cell, and all the complicated changes by which the various tissues and organs of the body are formed from it, after it has been fertilized, are the result of two general processes, viz., segmentation and differentiation of cells. Thus, the fertilized ovum undergoes repeated segmentation into a number of cells which at first closely resemble one another, but are, sooner or later, differentiated into two groups: (1) somatic cells, the function of which is to build up the various tissues of the body; and (2) germinal cells, which become imbedded in the sexual glands—the ovaries in the female and the testes in the male—and are destined for the perpetuation of the species. 2 Having regard to the main purpose of this work, it is impossible, in the space available in this section, to describe fully, or illustrate adequately, all the phenomena which occur in the different stages of the development of the human body. Only the principal facts are given, and the student is referred for further details to one or other of the text-books 1 on human embryology.