499 
arguments adduced by me for this second supposition, I here 
only will mention that formerly already Ll pointed out the 
possibility to prove or verify it by experiment. For in the 
centre of the apical plate the animal pole of the egg is found, 
characterised by the fact that there the extrusion of the polar 
bodies took place and the two first cleavage furrows crossed each 
other. A similar relation may now be expected to exist between the 
cerebral plate and the animal pole in the vertebrates, where the 
history of the animal pole cannot be traced with equal certainty, 
since the polar bodies do not stay there and the cleavage is not of 
such a kind that a detinite point can be kept in view. Pricking 
experiments should bring certainty here. 
If the cerebral plate of the Craniota is really homologous with 
the apical plate of the annelid (and mollusc) larva, it may be 
expected that we shall find back the animal pole on the cerebral 
plate. On closer examination it appears that this conclusion formerly 
drawn by me (1913 b) cannot be strictly correct. For a glance 
at the schemes then given of the structure of annelid, amphioxus 
and eraniote shows at once that in the latter not only the cerebral 
plate must be furnished by the apical plate, but also, just as in 
ihe former two, the ectoderm covering the prostomium. Hence the 
cerebral plate can only originate from part of the apical plate and 
moreover this part must, according to the scheme mentioned, be the 
half of the top-plate contiguous to the stomodaeum (in casu the 
medullary canal), which consequently is the oral half in the Annelida, 
but the aboral half in the Chordata, where the old mouth loses its 
frnetion and a new one breaks through at the opposite side. The 
other half then furnishes the ectoderm of the prostomium, not only 
ventrally, but also — the ectoderm cells during the closing of the 
cerebral plate evidently being shoved upwards on either side — 
dorsally, over the cerebrum. Now if the cerebral plate can be formed 
only from the posterior half of the apical plate, from the part where 
also with Annelids and Molluses the rudiment of cerebral ganglia and 
eyes is found, it may be expected that the animal pole will be found 
back not so much on the cerebral plate as either on or just in 
front of its anterior border, i.e. on the transverse head-foid. A 
glance at the three above-mentioned schemes given by me shows 
plainly that there is every reason to expect the animal pole at the 
level of the neuropore of the Craniota. To prove this experimentally 
would not only afford a verification of my theory, which reached 
this conclusion by a quite different train of reasoning, but would 
compel also those who attach little value to such theories on the 
DN) 
