GASTRULATION AND EARLIEST DEVELOPMENT 193 



4. The displacement of the endodermic area from opposite 

 the animal pole to the future dorsal side in the eggs 

 of lower Vertebrates is to be explained by a similar 

 displacement to the ventral side during the early 

 development of Annelids. 



5. The caudad-eccentric closure of the blastopore, typical 

 for Chordates, is a result of the interference of the 

 contraction of the blastopore border with the backward 

 shifting of the blastopore as a result of the elongation 

 of the medullary tube — former stomodaeum — which 

 in ontogeny occurs before this tube has been formed. 



Relation of anus and blastopore. — We shall pass now to 

 the question of the relation of the anus to the blastopore, 

 yet another question of Vertebrate embryology on which the 

 greatest uncertainty and confusion reign and which indeed, 

 as I hope to show, could hardly have been solved without 

 the aid of my theory. 



The statements made by the numerous investigators on 

 this subject are so divergent that it must be very difficult 

 for any one who can not judge from personal experience 

 to form a sound opinion. I shall try to show that the 

 application of the principles of my theory on the origin of 

 Vertebrates will once more serve to furnish us with the 

 solution of an old problem which has been resuscitated, 

 especially, by Grobben's (1900) classification of the animal 

 kingdom. In the first place the different views and results 

 of former investigators may be briefly reviewed. We shall 

 confine ourselves mainly to the Amphibian egg, in which 

 a relation between anus and blastopore was noticed for 

 the first time. Anurans and Urodelans will be treated 

 separately because, as I can confirm from my own invest- 

 igations on Rana esculenta and Amblysioma tigrinum, these 

 two groups exhibit a notable difference in the relation of 

 the anus to the blastopore. We shall begin with that group 

 on which the first observations were made, the Anurans. 



Different views on Anurans. — BALFOUR (1881), in his 

 Text-book, gives a description of the origin of the anus, 

 based mainly on the figures of GOETTE (1875) for Bom- 

 binator igneus and his own investigations on Rana 

 temporaria where the anus breaks through somewhat earlier 

 than appears to be the case in toads generally. The blas- 

 topore passes into the neurenteric canal and the anus 

 eventually arises at the bottoni of a diverticulum of the 



13 



