Chap. VI.] 



GENERAL EMBRYOLOGY. 



95 



portional to the concentration of the protoplasm. At the close 

 of the segmentation of the frog's egg (Fig. 34, i.) the ovum 

 forms a blastosphere containing 

 an excentric segmentation cavity, 

 roofed over with several layers of 

 small cells, and resting on a con- 

 tinuous mass of larger yolk-con- 

 taining cells. 



(2.) In the rabbit the segmen- 

 tation is complete and nearly 

 regular, but presents some pecu- 

 liar features. Two processes seem 

 to go on together. In addition 

 to the formation of a blastula 

 with a segmentation cavity, there 

 goes on a slipping inwards of 

 some of the cells to fill up this 

 cavity. And since the latter pro- 

 cess keeps pace with the former, 

 the cavity is filled up as rapidly 

 as it is produced. Thus at the close of segmentation there is 

 an outer layer (pu. 9 Fig. 34, ii.) of clear cells surrounding, except 

 at one spot (which we may term the pseudo-llastopore, v. l.\ an 

 inner mass of granular cells (In.}. 



(3.) In the fowl, owing to the enormous quantity of food 

 yolk and the consequent concentration of the protoplasm in the 

 germinal disc, the segmentation is not only very irregular, but 

 also incomplete or partial (meroblastic). That is to say, the 

 planes of division do not extend throughout the ovum, but are 

 confined to that comparatively small area of its surface known 

 as the germinal disc. The first two furrows are at right angles, 

 but do not cross each other quite in the middle of their length. 

 Four more radial furrows follow intermediate between these, 

 thus dividing the germinal disc into eight somewhat irregular 

 cells. The next furrow is irregularly circular (or polygonal), 

 giving rise to eight smaller central and eight larger peripheral 

 cells. Henceforth the central segments divide more rapidly 



v/ 

 Fia. 33. CLEAVAGE IN FROG'S OVUM. 



i.-iv. Stages with 2, 4, 8, and 16 blasto- 

 meres. v. A later stage when the smaller 

 blastomeres cover half the ovum. vi. A 

 still later stage when the smaller blasts- 

 meres have enveloped the whole ovum, 

 except at one spot, the blastopore, IL 



