210 THE ANCESTRY OF VERTEBRATES 



lary folds do not, as in Anurans, unite, but leave an 

 opening. We have seen that there is a passage from the 

 medullary tube as well through the anus to the exterior 

 as through the neurenteric pore into the archenteron. 

 Afterwards this is more or less obscured by the fact that 

 the medullary folds are in such close contact caudally that 

 there is here no lumen, no medullary canal (fig. 8, 

 plate) — just as in the frog (fig. 5) — and that accordingly, 

 as in the frog, the slit-like neurenteric pore would become 

 wholly virtual if the rear part did not remain open as the anus. 



Thus only the anterior part of the slit becomes virtual and 

 hence the statement of several authors concerning Urodelans, 

 Dipnoans and Petromyzontes, that the blastopore passes 

 into the anus and a neurenteric canal is wanting, is to 

 be explained. The apparent contrast between Anurans and 

 Urodelans has thus found a solution. It would cause us 

 no surprise if in an Anuran a similar condition were observed 

 as appears to prevail in Urodelans, nor would the reverse 

 case — the difference between them not being fundamental 

 but only graduated, it would not be impossible that in one 

 species at one time the first, at another the second case 

 might be realized (comp. De Lange and ISHIKAWA on 

 Megalobatrachus !) 



Formation of the tail. — I have spoken above of the 

 caudad movement of the neurenteric pore = blastopore 

 stopping in front of the anus. In reality, however, there 

 is no question of stopping. Although the anus, when it 

 has been reached by the cardiac =i neurenteric pore, seems 

 to afford an insurmountable obstacle for Ihe further back- 

 ward growth of the stomodaeum = medullary tube, the 

 activity of the periporal growing zone has not yet come 

 to an end when the perianal growing zone has stopped 

 working '1 here being no room, however, within the soma for 

 further extension, a protuberance of the body wall injront of 

 the anus results. Into this the stomodaeum = medullary tube 

 grows out. This protuberance is the tail-knob 

 (fig. 44, c). Thus we see the tail of Vertebrates originating by the 

 fact of the periporal growing zone continuing its activity 

 after the perianal has stopped. In this way the position of 

 the anus in Vertebrates is not terminal, as in Annelids, but 

 at the root of the tail which overgrows it and which owes 

 its origin simply to the presence of the anus. Phyloge- 

 netically we have to imagine that the longitudinal growth 



