204 



DEVELOPMENT OF FISHES 



is deeply cut by the furrows ; the yolk area, however, only 

 superficially ; the shallow furrow of the first cleavage on 

 the yolk hemisphere now passes through the lower pole ; 

 the second cleavage, passing downward, has made a shal- 

 low groove extending half-way between the rim of the 

 germ area and the lower pole of the &^^. It is the great 

 amount of yolk in the lower hemisphere that retards the 

 cleavage of the blastomeres. In Fig. 252 the entire 

 germ area has become subdivided into a mass of small 

 cells, while the large, irregular blastomeres of the yolk 

 hemisphere are separated only by superficial furrows. 

 This stage, the blastula, is seen in section in Fig. 253: 

 the yolk, unsegmented, occupies the lower hemisphere ; 

 the germ area contains a segmentation cavity, SC, with 

 a roofing of small cells, and a floor of irregular cells half 

 engulfed in a deep, underlying zone transitional between 

 germ and yolk. 



An early gastrula is seen in Fig. 254 : the more rapid 

 multiplication of the cells of the germ region has given 

 rise to a down-reaching cap of cells, whose boundary is 

 here sharply marked off from the large and imperfect yolk 

 cells of the lower hemisphere. At BP, the rim of the cell 

 cap, or blastoderm, is sharply distinct from the yolk ; it is 

 the dorsal lip of the blastopore ; the remaining portion of 

 the rim is, generally speaking, the remainder of the rim 

 of the blastopore ; more accurately it is the circumcres- 

 cence margin of Hertwig. The late gastrula of Fig. 255 

 shows the greatly increased extent of the blastoderm : its 

 margin is continually reducing the size of the blastopore, 

 BP ; on its dorsal lip at HE, the outline of the embryo 

 is appearing. A sagittal section of this stage (Fig. 256) 

 shows at BP the dorsal, and at VL the ventral, lip of the 

 blastopore ; at YP the yolk material appears at the egg's 



