GASTRULATION AND EARLIEST DEVELOPMENT 213 



the relative strength of the two growing zones that in 

 Urodelans the blastopore is pushed bacic to the anus before 

 the tube-formation, in Anurans on the contrary it does not 

 reach it till alter the tube-formation. The beginning of the 

 latter evidehtiy is determined again by the end of the 

 gastruiation, just as in Protostomia the stomodaeal tube is 

 formed directly after gastruiation In Selachians, where the 

 accomplishment of the gastruiation is so much retarded by 

 the great yolk- wealth, urogenesis actually sets in before 

 the tube-formation, the neurenteric canal thus originally 

 being an open groove known as the sulcus neurentericus. 

 Thus there are three possibilities: 



1. when the gastruiation is accomplished and the tube- 

 formation ensues, the receding blastopore has not yet 

 reached the (future) anus. Immediately after the closure 

 of the neural folds we then have the case of fig. 44 a, 

 realized in Anurans. 



2. the action of the periporal growing zone being stronger 

 than in the foregoing case, the blastopore has reached the 

 (future) anus when the gastruiation has finished and 

 the tube-formation occurs. Immediately after the accohi- 

 plishment of the latter we then have the case of fig. 44 b, 

 realized in Urodelans. Of course this stage is passed 

 through by Anurans also before the tail-formation begins. 



3. the yolk has so much retarded the end of the gastruia- 

 tion that the neurenteric canal has already passed the 

 anus when it has finished and the tube-formation 

 ensues. Immediately after the latter we get the condition 

 of fig. 44 c, as is found in Selachians. This figure, 

 however, represents at the same time the tail-formation 

 in Urodelans and Anurans. 



While I feel that the application of my theory has thus 

 thrown light on a number of obscure problems, the facts 

 and results recorded above afford to my theory yet further sup- 

 port of no inconsiderable value. In contemplating fig. 44 we 

 see, as it were, pass before our eyes the growing out of the sto- 

 modaeum and the travelling backwards of the poruscar(//ac«s 

 or neurentericus, which are demanded by my theory but which 

 only after a careful analysis are to be recognized in the early 

 ontogenetic processes which then, however, prove to be 

 explained by them in such a surprising manner 



Mesoderm of Vertebrates. — Before closing this chapter 

 we must devote at least a few words to the question of 



