196 



DEVELOPMENT OF FISHES 



Fig. 223 is a pale-coloured circular membrane of about a 

 half inch in diameter lying on the surface of the egg. 

 Sectioned at an earlier stage (Fig. 222) the blastoderm is 

 seen to present the following contrast to the blastula of 

 Fig. 221 : the floor of the segmentation cavity has flattened, 

 and a sharp rim forms the outline of the blastoderm ; at 

 one side this rim is seen to protrude ov 7 er the yolk mass, 

 leaving a narrow, fissure-like cavity between. This stage 

 is identified as the gastrula ; the fissure-like cavity, the 

 coelenteron ; its marginal blastoderm, the dorsal lip of the 

 blastopore ; its ventral lip, the entire yolk mass. 



The growth of the embryo's form takes its origin at the 

 blastopore's dorsal lip. In Fig. 223 the rim of the blasto- 

 derm is seen indented near the point CF, and its thicken- 

 ing at this region becomes more and more marked in 

 subsequent stages ; on the other hand, the anterior por- 

 tion of the blastoderm, growing continually on all sides, 

 becomes excessively thin, flattening itself tightly to the 

 yolk, and reducing the segmentation cavity to the small 

 area indicated at SC. The growth of the embryo in the 

 mid-region of the blastopore's dorsal lip may next be 

 followed in the stages, Figs. 224, 225, 226. The inden- 

 tation of the rim may thus be seen to assume a creese- 

 like thickening, thrusting forward its blunt end, the head 

 eminence, HE, over the blastoderm ; at the points CF, 

 the tail eminences, the rim of the blastoderm is thick, 

 protruding, appearing to be pressing together in the 

 median line, and causing the body of the embryo to be 

 actually pushed into form and thrust above the level of 

 the blastoderm. In Fig. 225 the sides of the embryo are 

 separated dorsally by a deep groove, the medullary furrow, 

 the future canal of the central nervous system. In Fig. 

 226 this is seen at a more advanced stage ; its hinder 



