GASTRULATION AND EARLIEST DEVELOPMENT 



191 



(p. 281) that "une partie importante de la plaque medullaire 

 siege en arriere du blastopore", as follows from fig. 2 (cf. p. 9). 

 This seal-ling or crescent is evidently to be derived by 

 one-sided development from a ring of uniform breadth as 

 represented by the stomodaeal rudiment in Protaxonia. 



During the contraction of the blastopore this ring-shaped 

 rudiment of the medullary tube undergoes a change in 

 shape as indicated in figs. 36 / and 39 We see here the 

 backward growing out of the stomodaeum of Anneldsinto 

 the epichordal neural tube of Chordates projected as it were 

 on a plane, ihis being the dorsal side of the embryo Thus in 

 ontogeny it is not the neural tube itself that grows out but 

 its rudiment, before the tube has been formed. This may at 



med j. 

 (pneurenK) 



a. b. c. d. 



Fig. 39. Formation of the epichordal part of the medullary tube in 

 Chordates In d the medullary folds are closing. 



first appear a little strange and it might easily be 

 supposed that it is the neurotroch of the Annelid which 

 has simply sunk under the surface and that in this way 

 the ciliated neural tube has been formed. By this assump- 

 tion, however, the fate of the blastopore, its backward 

 movement and its conversion into the neurenteric canal, 

 would not be accounted for Moreover, we shall find our 

 conception confirmed when considering the relation of the 

 anus to the blastopore and the formation of the tail. 



As we have seen there is a certain genetic relation 

 between the neurotroch and the stomodaeum in Annelids 

 (cf. fig 37) and there is reason to consider the former as 

 an obliterated part of the latter As both consist cf ciliated 

 and vacuolated cells, there is also a certain histological 

 agreement. If now the blastopore moves backwards in 

 Chordates it is quite possible, and even probable, that 

 the cells of the neurotroch are reincorporated into the 

 material for the stomodaeum, now the medullary tube. Then 

 there appears to be still a certain relation between the 



