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 egg. 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 A&P, 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 AZ, the outline of the embryo 
is appearing. A sagittal section of this stage (Fig. 256) 
shows at BP the dorsal, and at VZ the ventral, lip of the 
blastopore ; at YP the yolk material appears at the egg’s 
