239 
cause of this difference we must, | 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 development 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 fusca both the ventral and the dorsal 
side encircle 180%, the anus accordingly being situated 
directly opposite the animal pole or anterior end of the 
embryo (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. 
e precocious longitudinal growth of the meduliary 
plate ín 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. lt 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 
