CHAP. X.] 



THE DEVELOPMENT OF THE CA1. 



319 



nuclei, and the protoplasm of the ovum also divides, arranging itself 

 around each secondary segmentation nucleus. The two halves are 

 not quite equal in size. One, larger and more transparent (Fig. 

 141, B, e) is the ectodermic or epiblastic sphere, the other (i) is the 

 endodermic or hypoblastic sphere. Each sphere again subdivides, 

 and the same process is repeated with each subdivision, there being 

 thus four spheres, or cells, derived from the primitive epiblastic 

 sphere, and four from the primitive hypoblastic sphere. One of 

 these latter now assumes [a, central position (Fig. 141, C). The 

 process of division is then continued, but unequally, the cells derived 

 from the epiblastic sphere dividing more rapidly, and therefore be- 

 coming smaller than these from the hypoblastic sphere. Moreover 



A. Impregnated ovum, with a single segmenta- 

 tion nucleus. 

 n. External envelope. 

 z. Zona pellucida, with spermatozoa. 

 n. Segmentation nucleus. 

 y. Protoplasm or yelk of the ovum. 



B 



Fig. 141. YELK SEGMENTATION. 



B. Ovum, with the yelk divided into two 



cleavage cells, each with its nucleus. 

 e. Epiblastic cleavage cell. 

 i. Hypoblastic cleavage cell. 



C. Fourth segmention into eight cells four 



epiblastic and four hypoblastic. 



the epiblastic cells come to surround and enclose the hypoblastic 

 cells, save, perhaps, at one point where the hypoblastic cells may 

 appear at the surface (Fig. 142). The mass of cells, however, forms 

 a solid whole. There is no central cavity. Soon, however, a clear 

 space appears, and liquid forms between the mass of hypoblastic cells 

 and the epiblastic cells (Fig. 143, A), save at one part where the 

 two sets of cells adhere together. This process rapidly continues as 

 the ovum grows, the interspace becoming wider and wider, till at last 

 we have a relatively large sphere of very small epiblastic cells, against 

 the inside of one part of which the aggregation of small hypoblastic 

 cells is flattened out, so that we have a double stratum of cells, consist- 

 ing of a single layer of hypoblastic cells beneath a single layer of epi- 

 blastic cells (Fig. 143, B). In other words, we have two cellular 

 membranes an epiblast and a hypoblast the epiblast investing 

 the whole ovum within the vitelline membrane, the hypoblast under- 

 lying only a portion, though a considerable portion, of the epiblast. 

 The cellular membrane thus investing the ovum, and derived from 

 the segmentation of the yelk, is called the blastoderm, and the 

 whole structure is the blastodcrmic vesicle. That part where the two 



