GASTRULATION AND EARLIEST DEVELOPA\ENT 



209 



yet a little further (fig. 44 b), a secondary relation between 

 neurenteric pore (blastopore) and anus results ^) The anus 

 now opens to the exterior through the hindmost extremity 

 of the medullary tube. From the medullary canal there is a 

 passage through the anus to the exterior as well as through 

 the neurenteric pore into the archenteron, and from the 

 archenteron through the neurenteric pore and the anus to the 

 exterior. In ontogeny this will result in the non closing of the 

 medullary folds over the rear end of the slit-like blastopore, 

 and in the leaving of an opening, viz. the anus. Possibly 

 they will develop slightly at both sides of the rear part of 

 the blastopore under the influence of the formation of the 

 a. b. c. 



Fig. 44. (cf. fig. 11, p 220). Diagram of the relation between aaus 

 and blastopore, and of the tail-forming (cf. p. 213). 

 a at the moment of the closure of the neural folds is Anurans, 



b n n „ n „ n „ r, n n n UrOdClanS, 



C r, r, „ r, n n „ n , n n SclaChlanS, 



at the same time representing the growing out of the tail 



in Amphibians. 



a anus, p n neurenteric pore; the entoderm is dotted. 



anus at this point, or possibly not. If we imagine things 

 very much enlarged and if we look through the anus into 

 the interior, we shall see the slit-like blastopore (neuren- 

 teric pore) in the background, though its rear end, under 

 the influence of the formation of the anus, will probably be 

 slightly widened. With ihis conception the facts stated by us 

 in Urodelans so perfectly agree that it seems hardly possible 

 to doubt the accuracy of this interpretation. We see in the 

 Axolotl the blastopore move backwards to the place where in 

 Anurans the anus breaks through. We see over that place, 

 that is over the rear end of the blastopore, that the medul- 



^) In a longitudinal section as in fig. 44 the conditions at first 

 sight might appear in fig. 44 b radically different to those in 44 a. 

 In an elevation, however, the agreement between them will be evident. 



14 



