GASTRULATION AND EARLIEST DEVELOPMENT 157 



Fate of the animal pole. — In the first place they concern 

 the problem as to what becomes of the animal pole 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) I suggested that the cerebral 

 plate is the homologon of the apical plate of the trocho- 

 phora-larva, i e. 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 tgg 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 lo 

 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 this 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 and the anterior end of the 

 embryonic rudiment would not only be interesting in 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 



