190 THE ANCESTRY OF VERTEBRATES 



Interpretation of the eccentric closure of the blastopore. — 

 The second process mentioned above, the rostrad-eccentrical 

 c osure of the blastopore, is not found in the frog egg; 

 on the contrary, the closure proceeds caudad-eccentrically, 

 I see in this caudad-eccentrical closure a result of the inter- 

 ference of the contraction o? the blastjporic border with 

 a backward movement of the blastopore, following directly 

 from my theory on the homology of stomodaeum and 

 epichordal neural tube in Annelids and Vertebrates. As 

 a result of the strong elongation, which we must assume 

 the stomodaeum of Annelids undergoes in order to be 

 transformed into the medullary tube of the Vertebrates, the 

 entrance to the stomach, the cardiac pore, into which the 

 blastopore passes, must mo\^e backwards over the whole 

 length of the body — it moves still further, as we shall 

 see (formation of the tail) — to become the neurenteric 

 canal (also representing the former blastopore). This 

 backward movement is performed in Chordates in anticipa- 

 tion of t e foimation of a tube and during the contraction 

 of the blastopore border, thus interfering with the gastrulation 

 process. In this way the final, narrowed, blastopore is 

 carried back to the place where it was originally found in 

 Protaxonia. viz: diametrically opposite the animal pole. 

 Whether this caudad-eccentrical closure of the blastopore is 

 performed by concrescence or not is here of no importance; 

 as stated earlier, I do not believe that concrescence, in 

 Amphibians at least, occurs to any considerable extent, and 

 if it plays a role in more yolk-laden eggs, evidently no 

 primary significance should be attributed to it. The medullary 

 plate 'n stage e (fig. 36), just as the rudiment of the 

 stomodaeum in fig. 36 b, su. rounds as a crescent the 

 anterior border of the blastopore. This conception is reached 

 for Amphioxus, e. g. by KORSCHELT and HEIDER in the last 

 edition of their "Lehrbuch" (1910, p. 435), and especially 

 for Ascidians by VAN Beneden and JULIN (1884, 1887), 

 Castle (1896) and CONKLIN (1905) in their cell- lineage 

 investigations. VAN BENEDEN and JULIN (1887, p. 259) state 

 that the rudiment of the medullary tube surrounds the 

 blastopore, presenting "la forme d'un anneau ou plus 

 exactement d'une bague chevaliere, Tanneau elargi en une 

 plaque en avant du blastopore se retrecit progressivement 

 sur les cote's et se reduit a son minimum en arriere de 

 cet orifice." In contradiction to CONKLlN they emphasize 



