198 THE ANCESTRY OF VERTEBRATES 



greater resemblance to fig. 5, where the medullary folds 

 have coalesced, than the median one of fig. 3. 



At the bottom of the body the anus has broken through, 

 the ventral blastopore lip as a consequence seems to have 

 suddenly vanished. The blastopore itself has been overgrown 

 by the medullary folds. In the hindmost part of the medullary 

 tube these folds have applied themselves so closely one to 

 the other that the lumen of the tube is not continued between 

 them and only a virtual neurenteric canal can be spoken of. 

 From figs. 4 and 5 and from the study of whole eggs it 

 appears quite evident that the medullary folds unite over 

 the blastopore and that the anus breaks through a little 

 distance behind it at the bottom of the small depression 

 indicated in fig. 40 c. 



I should like to emphasize a peculiarity which has only 

 be noted by ERLANGER (1890), especially in relation to 

 what we shall find in Urodelans. In the short time that 

 passes between the stages of fig. 1 and fig. 3 (plate II), 

 the distance between blastopore and future anus diminishes 

 slightly; in other words, if we take the place of the future 

 anus as a fixed point, the slit-like blastopore moves a little 

 backwards towards it. Thus the ventral blastopore lip 

 in median sections is not only getting thinner, owing to 

 the appearance of the groove between blastopore and 

 anus, but also somewhat shorter. To this point we shall 

 revert later. 



Different views on Urodelans. — Let us pass now to the 

 Urodelans. The small extension of the ventral ectoderm 

 and the strong development of the dorsal parts is here 

 characteristic in the early stages of development, the 

 foundation of the embryo accordingly encircling the egg 

 over considerably more than 180°. The Urodelans have 

 this peculiarity in common with the Dipnoans and Petromy- 

 zontes of which the earlier stages of development, externally 

 as well as in sections, exhibit a striking similarity to those 

 of Urodelans. 



According to SCOTT and OSBORNE (1879) the blastopore 

 of Triton is overgrown by the medullary folds and becomes 

 the neurenteric canal. SEDGWICK (1884), in his well-known 

 article on the origin of metamerism, writes concerning 

 Triton cristatus: "in this animal the blastopore appears not 

 to close, but to persist as the anus" and his pupil ALICE 

 JOHNSON (1884) verified this by sections. A neurenteric 



