600 AMPHIBIA, 2 
below. Within there is a small segmentation cavity. Since 
the presence of yolk acts as a check on the activity of the 
protoplasm, we can understand why the smaller cells continue 
to divide much more rapidly than the large yolk-containing 
cells, and so how the smaller epiblastic cells gradually 
spread over the egg, covering in the larger ones. At one 
point, where upper and lower cells meet, a groove is 
formed. This groove represents the dorsal lip of the 
blastopore. It becomes crescentic and moves as a whole 
down over the large yolk-cells. Invagination of the small 
cells of the upper hemisphere goes on rapidly all round this 
crescentic groove, and the archenteron is thus formed. The 
horns of the cres- 
cent meetata point 
near the lower pole 
of the egg to form 
the ventral lip of 
the blastopore. 
The _ blastopore 
now becomes re- 
duced, by the in- 
growing of its mar- 
gins, to a small 
Fic, 325.—Longitudinal vertical section of . a 
frog embryo, shortly before closure of blasto- = C1 cular area which 
pore.—After Ziegler’s model and Marshall. appears white, the 
FB., fore-brain; EC., ectoderm; N., notochord; SC. j 
canal of spinal cord ; NVE., neurenteric canal; B. colour being due 
blastopore; J7., mesoderm cells; ¥., Yolk-laden toa plug of yolk- 
cells; 4ZV., mesonteron; /., beginning of pituitary cells which almost 
invagination. 
obliterates its 
opening. The whole egg now rotates backwards through 
a little more than a right angle, so that the blastopore is 
carried up into the position previously occupied by the first 
trace of its dorsal lip. The blastopore now marks the 
posterior end of the embryo. The archenteron has by this 
time greatly enlarged, and has pushed the segmentation cavity 
almost out of existence. The embryo elongates slightly, but 
the mass of yolk-laden cells which lie on the floor of the gut 
prevents the body acquiring at once the fish-like shape. 
Along the mid-dorsal line the usual neural plate forms the 
medullary canal. At the posterior end this communicates 
with the archenteron for a time by the neurenteric canal. 
