96 OUTLINES OF CHORDATE DEVELOPMENT 



Since the blastula retains its spherical form it is evident 

 that the downward extension of the germ ring must displace 

 the yolk cells in some direction, and as a matter of fact this 

 displacement is evidenced by the elevation of the floor of the 

 blastocoel. As the animal cells push downward the internal 

 yolk cells rise till the floor of the blastocoel becomes first flat, 

 and then convexly arched; at the same time the cavity widens 

 somewhat so that in section it appears broadly crescentic 

 (Fig. 32, A, B). 



The first evidences of gastrulation now appear so that this 

 stage must be taken to mark the completion of the blastula. 

 We may state the characteristics of the fully developed blastula 

 as follows. The completed blastula is spherical, in volume 

 about one-fifth larger than the ovum, and bilaterally symmet- 

 rical; this bilaterality is accompanied by antero-posterior 

 differentiation, and is indicated by the greater thickness of 

 the anterior wall of the segmentation cavity, and by the more 

 ventral extension of the pigmented cells on the posterior side, 

 i.e., the side marked by the gray crescent in the egg. The 

 small cells of the upper pole form the thin roof and thicker 

 sides of the eccentric blastocoel; they are in two quite distinct 

 sheets an outer layer of compactly arranged cells forming a 

 distinct epithelium known as the superficial or epidermal layer, 

 and, lining the blastocoel, a deeper or " nervous" layer of 

 irregularly and loosely arranged cells, gradually increasing in 

 thickness from the pole toward the base of the blastocoelic wall, 

 about at the level of the equator of the blastula. Just at and 

 below the equator actively dividing cells have accumulated 

 from the whole upper pole region. Since the fate of these 

 cells is to form the chief axial parts of the embryo this region 

 is called the germ ring, although it lacks the distinctness of the 

 germ ring as it is finally found in some other forms (e.g., 

 Teleosts). This region seems comparable with a crescentic 

 group of actively dividing cells having a corresponding position 

 and function in the blastula of Amphioxus (Fig. 6). The floor 

 of the blastocoel is formed of the larger vegetative cells which 

 form practically the lower half of the blastula; these are com- 



