ORIGIN AND STRUCTURE OF THE HEAD 53 



cause of this difference we must, 1 believe, recur to the 

 same peculiarities of the early development of both groups, 

 from which the difference in the relation of the anus 

 and the blastopore is to be explained (cf. last chapter). In 

 Urodelans we see a very strong develc^ment of the dorsal 

 parts, especially of the medullary plate which, as a con- 

 sequence, in an embryo like that of fig. 8 (plate) encir- 

 cles much more than 180*^ of the circumference of the 

 egg, the ventral being, as it were, compressed to much less 

 than 180". In Rana fasca both the ventral and the dorsal 

 side encircle 180°, the anus accordingly being situated 

 directly opposite the animal pole or anterior end of the 

 em.bryo (cf. fig. 35). In Rana esculenta the opposite condition 

 to what we find in the Axolotl is realized, the ventral 

 side encircling somewhat more than 180°, the medullary 

 plate correspondingly less. 



The precocious longitudinal growth of the medullary 

 plate in Urodelans seems to me to explain not only the 

 difference in the relation of the anus to the blastopore in 

 Urodelans and Anurans (cf. last chapter), but also the difference 

 in the closure of the cerebral plate. By this precocious 

 lengthening the rear extremity of the medullary plate, 

 with the blastopore, is pushed backwards and the anterior 

 extremity is pushed forwards Thus both extremities 

 approach each other on the ventral side, forcing the latter 

 to extend more in a lateral direction, as shown readily 

 by the study of surface views of Axolotl-eggs (fig. 41). 



Now, according to the conceptions reached by us in this 

 article, the closure of the cerebral plate consists in the 

 folding in of that part of the apical plate of the trocho- 

 phore, or the epithelium of the prostomium of the Annelid, 

 which is situated between the animal pole and the mouth. 

 The rest of the apical plate is situated accordingly in the 

 form of a crescent in front of and round the cerebral plate, 

 which reaches forward to the animal pole, the centre of 

 the apical plate. As shown by fig. 1 (plate) the extra-cere- 

 bral part of the apical plate in Rana is thickened in the 

 same way as the cerebral plate itself and shows a similar 

 distinction of a basal and a superficial layer. It closes over 

 the cerebral plate and gives rise to the ectoderm of the snout. 

 This closing in Rana is performed both from the anterior 

 and from the lateral sides. The fusion of the lateral folds 

 produces the dorsal suture which is the prolongation of the 



