160 THE ANCESTRY OF VERTEBRATES 



head-fold, opposite the place where the blastopore closes. 



The results described above point to the general pre- 

 valence of a relation between the animal pole of the egg 

 and the anterior border of the cerebral plate, as might be 

 expected from my theory. For the sake of completeness I 

 must mention here that HELEN DEAN KING (1902) in Bufo 

 and EycleshymeR (1902) in Nectiirus concluded from 

 similar pricking experiments that in these forms the 

 animal pole is found some distance in front of the 

 transverse brain-fold and that the latter lies even halfway 

 between the animal pole and the equator. However, 

 it seems to me that the evidence yielded by these expe- 

 riments is not so conclusive as to preclude the possibility 

 that on re-examination these forms also might turn out 

 to conform with the rule found to be valid for such closely 

 allied species. 



Acrania and Craniata. — In the foregoing chapter we 

 have pointed already to the fact that the relation of the 

 animal pole to the cerebral plate in Amphioxus is a different 

 one from that in Craniata. In Amphioxus, as shown by 

 CerfoNTAINE's (1906) figures, the place of the animal 

 pole is often indicated until the gastrula-stage by the second 

 polar body which remains fixed to the egg. Here also the 

 closure of the blastopore occurs nearly opposite the animal 

 pole, but a comparison of a gastrula where the polar body 

 is still present, as represented in fig. 12, with a somewhat 

 older stage, with a neuropore but where the polar body 

 has been lost (fig. 5), renders it quite evident that, if in the 

 latter the polar body were still present, it would be found 

 at a considerable distance in front of the neuropore. In 

 both Acrania and Craniata the animal pole lies at the 

 anterior end of the embryonic rudiment, in corresponding 

 places with regard to the main axis of the embryo and to 

 the place where the blastopore closes (with regard to the 

 latter circumstance exception must be made for very yolk- 

 laden eggs). The neuropore in Craniata however lies ter- 

 minally and close to the animal pole while in Acrania it 

 is situated dorsally, a good distance away from the animal 

 pole. This is one of the circumstances which, in the fore- 

 going chapter, has induced me to conclude that the prae- 

 chordal part of the brain of Craniates is not yet present 

 in Acrania, In reality, however, 1 first made this conclusion 

 from other reflections (1913) and only afterwards, by the 



