204 THE ANCESTRY OF VERTEBRATES 



with a germinal disc, or in eggs that are to be derived 

 from yolk- laden ones, such as those of Mammals. 



As stated earlier, 1 shall not absolutely deny that concresc- 

 ence ever plays a part in Vertebrate gastrulation, particularly 

 in yolk-laden eggs. It seems to me beyond doubt, however, 

 that its role is a much more subordinate one than the well- 

 known doctrine of His assumes. For Amphibians the pricking 

 experiments described above have shown that there can 

 be no question' of the whole dorsal side of the embryonic 

 rudiment arising by concrescence of the blastoporic lips. 



For concrescence at the hind border of the tilastopore, 

 as assumed by ROBINSON and ASSHETON, still less evidence 

 can be adduced. The groove between the slit-like blastopore 

 and the anal pit does not become gradually longer, as 

 might be expected in this case, the anal pit moving away 

 from the ventral border of the blastopore, but on the contrary 

 it only gradually becomes more distinct ai d at the same 

 time shorter, the blastopore approaching the anal pit. 

 Evidently it is not to be [considered as a concrescence- 

 seam, perhaps it may be compared to the groove 

 joining the two impressions made by two fingers pressed 

 near one another into a soft cushion. When ROBINSON 

 and ASSHETON (1891, p. 475) compare it with the primitive 

 streak e.g. of a bird, they encounter at once the difficulty 

 that in the latter it is formed from in front backwards, in the 

 frog egg, according to their assumption, from behind forwards. 



Three possibilities may be distinguished concerning the 

 relation between blastopore and anus in Vertebrates: 



1. there is a primary relation 



2. there is no relation 



3. there is a secondary relation. 



Primary relation between anus and blastopore. — The first 

 supposition mentioned above is now the one most widely 

 accepted. Even where in Anurans 2 seems to prevail, yet 

 it is assured that this is to be traced back to 1, since 

 what is found in Urodelans may be supposed to be valid 

 for Anurans also. Thus MAURER (1906) in HERTWiG's 

 Handhuch tries to trace back all the results for Chordates 

 to 1, though the evidence adduced can not always be called 

 convincing. Even in Amphioxus n^ relation between the 

 anus and the blastopore has as yet been established. 



The possibility of 1 is in no way precluded by my theory 

 which derives the Chordates, in opposition to Grobben, 



