EMBRYOLOGY 9 



The enormously complicated body of every vertebrate is derived 

 from a comparatively simple specialized cell, the egg or ovum. 

 This ovum must be fertilized by a still more specialized cell, the 

 spermatozoon, derived from the male. After this fertilization the 

 egg goes through an orderly but very gradual series of changes which 

 bring it continually nearer the adult condition. The phases of this 

 differ with different animals ; here only a generalized account will be 

 given, which is subject to modifications in the several groups, for an 

 account of which reference must be had to embryological text-books. 



The Segmentation of the Egg. — The first steps of the process are 

 the segmentation or cleavage of the egg, in which it divides again and 



Fig. 3. — Diagram of a typical 

 blastula with central segmentation 

 cavity. 



Fig. 4. — Diagram of a gastrula, 

 a, archenteron; b, blastopore; ec, ecto- 

 derm; en, entoderm; sc, segmentation, 

 cavity. 



again, until the single-celled egg is converted into a large number of 

 cells or blastomeres (fig. 2). The character of this segmentation is 

 modified accordingly as the egg is large or small, as it contains vary- 

 ing amounts of nourishment — deutoplasm or food yolk stored up for 

 the growing embryo. These same variations also affect the later 

 stages of development; the description given here follows the simplest 

 conditions. 



As a result of segmentation the egg is converted into a spherical 

 mass of cells in which a cavity appears, called the segmentation 

 cavity because it is formed during segmentation. It also has the 

 name archicoele as it is the first or oldest space to appear in the em- 

 bryo. This stage of the embryo is called the blastula (fig. 3). 

 Its cells at first show but little differentiation except in size. Next 



