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 over 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 ifself 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 C/, 
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 
