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Fate of the animal pole. — In the first place they concern 
the problem as to what becomes of the animal pol of the egg. 
The importance of an answer to this question, to which until 
now only little attention has been paid, has already been 
emphasized in the foregoing chapter. In the first edition 
of my theory (1913, p. 685) l suggested that the cerebral 
plate is the homologon of the apical plate of the trocho- 
phora-larva, ie. the surface of the prostomium of the adult 
Annelid, and concluded that the accuracy of this supposition 
could be verified by determining the place of the animal 
pole of the egg in the foundation of the embryo. In Annelids 
the animal pole is found in the centre of the apical plate, 
in Vertebrates as a consequence it could be expected to 
be located on the cerebral plate. Closer examination, how- 
ever, showed (1916, p. 499) that this conclusion in the 
present form could not be right since the apical plate or 
the surface of the prostomium of Annelids has to give rise 
not only to the cerebral plate but also to the ectodermal 
epiderm of the prostomium in Vertebrates. Originally 
this ectodermal material is situated only on the ventral 
side of the prostomium, the cerebral plate occupying the 
dorsal part, but after the latter has been folded in and 
has closed this ectoderm clothes the prostomium dorsally 
as well. Thus only part of the area corresponding to the 
apical plate can give rise in Vertebrates to the cerebral 
plate and this must be the dorsal half, in front of the 
former mouth which is now, as we have seen, the neuro- 
pore in Amphioxus, sometimes appearing as a provisional 
neuropore in Craniates. The other, ventral, half then gives 
rise to the epiderm of the prostomium, not only ventrally 
but soon after dorsally as well, over the brain. From thís it 
follows that the animal pole may be expected to be found 
again not so much on the cerebral plate but either on or just in 
front of its anterior border, the so-called transverse head- 
fold or brainfold. To state the general prevalence of such a 
relation between the animal pole ar.d the anterior end of the 
embryonic rudiment would not only be interesting Il itself 
but it would be of great value also as a crucial test of 
the accuracy of our conclusions and the assumptions on 
which they rest. 
Teleostean eggs. — The determinate cleavage and the 
regular arrangement of the cleavage-cells in Annelids renders 
it possible to trace the fate of the animal pole with great 
