507 
stage of Rana would moreover in their prospective significance 
approximately agree with the corresponding four cells of the eight- 
celled stage of Annelida and Mollusca, ie. with the so-called first 
quartet of micromeres, which in fact produces the top-plate. Still this 
supposition gives rise to considerable difficulties, which can only ‘be 
overcome by additional suppositions. So e.g. the place of the mouth, 
which in Craniota lies behind the first body-segment, represented 
by the praemandibular mesodermic segment. Now if the supposition 
mentioned were right we should have to admit that the anterior 
mesodermie segments, as is the case in Ampbioxus with the foremost 
point of the notochord, were pushed into the prostomium and that 
accordingly the mouth would also break through on the prostomium. For 
the olfactory grooves, which in Annelida lie on the border of prostomium 
and first segment, although occasionally a little way on the prostomium, 
a similar shifting would have to be admitted. This question seems 
for the present too difficult and too uncertain to be dealt with here. 
So we see that while the rudiment of the cerebral plate originates 
in the animal half of the eight-celled egg, the rudiment of the 
remainder of the medullary plate is found in the four vegetative 
cells, chiefly of course in the two dorsal ones. This rudiment has, 
when the blastopore appears, the shape of a crescent, the largest 
breadth of which is measured by the distance between mark b and 
the blastopore lip. When the lip of the blastopore moves 
backward this maximum breadth increases proportionally to the 
distance between mark and the rim of the blastopore. At the 
same time we may assume that just as the border of the blastopore 
goes on differentiating itself laterally from the cell-material there 
present, the crescentic rudiment of the medullary plate does the 
same, so that both horns of the crescent extend laterally backwards 
and finally almost join behind the blastopore. Meanwhile the possi- 
bility is granted that here a small cap remains, in regard to the 
eventual development of the anus from the posterior part of the 
blastopore. With the majority of more recent investigators | am of 
opinion that nothing pleads for concrescence taking place at the 
closure of the blastopore, unless at the very last when the blasto- 
pore occasionally assumes a pear-like shape, soon followed by the 
slit-shaped closure. I assume the caudally-excentrie closure of the 
blastopore to be derived from a concentric, or perhaps even a rostrally- 
excentric one, such as is found in Annelida, by interference of this 
latter with a caudal shifting of the blastopore, which follows directly 
from my theory on the origin of the medullary canal from the 
stomodaeum of the Annelida and which is observed beautifully in Amni- 
