164 
the relative strength of the two growing zones that in 
Urodelans the blastopore is pushed back to the anus before 
the tube-formation, in Anurans on the contrary it does not 
reach it till aiter the tube-formation. The beginning of the 
latter evidertly is determined again by the end of the 
gastrulation, just as in Protostomia the stomodaeal tube is 
formed directiy after gastrulation. In Selachians, where the 
accomplishment of the gastrulation 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: 
l. when the gastrulation is accomplished and the tube- 
formation ensues, the receding blastopore has not yet 
reached the (future) anus. lmmediately 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 gastrulation has finished and 
the tube-formation occurs. lmmediately after the accom- 
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 gastrula- 
tion that the neurenteric canal has already passed the 
anus when it has finished and the tube-fo:mation 
ensues. lmmediately 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 porus cardiacuis 
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 O 
