GASTRULATION AND EARLIEST DEVELOPMENT 203 



canal is not continued any further between them, as 

 MORGAN has already remarked. Hence the opinion of many 

 investigators that the medullary folds do not reach to the 

 blastopore and that there is no neurenteric canal. The 

 hindmost part of the neurenteric pore remains open as 

 ihe internal opening of the anus. The result is really that 

 the hind wall of the hindmost part of ihe medullary tube is 

 perforated by the anus, which in Anurans aiises closely 

 behind it, and this is caused by the circumstance that the 

 neurenteric pore, the former blastopore, in Urodelans has 

 travelled back so far that its rear end has reached the place 

 where in Anurans the anus breaks through. This is at the 

 same time the solution of the apparent contradiction 

 between Anurans and Urodelans in this respect. 



Different interpretations — The interpretation which until 

 now has been fairly generally accepted is that of SCHANZ 

 (1887), MORGAN (1890 1, ERLANGER (1890) and ROBINSON and 

 ASSHETON (1891) who contend that the place where the anus 

 in Anurans breaks through really represents the rear end of 

 the original wide blastopore, which has narrowed down by 

 concrescence of the lateral borders not only at the anterior end 

 from in front backwards, as postulated by HIS' concrescence 

 theory, but also at the posterior lip from behind forwards. 

 The longitudinal groove between the blastopore and the 

 anal depression in fig. 40 seemed to be an indication of 

 a raphe Thus the anus in Anurans would be closed only 

 temporarily and would not arise as an independent formation. 

 In this way ERLANGER assumed concrescence at the dorsal 

 as well as at the ventral blastopore border, ROBINSON 

 and ASSHETON only at the ventral border. The line. of 

 concrescence in both cases is compared to a primitive 

 streak However, a primitive streak, as ROBINSON and 

 ASSHETON remark, can be expected only behind the blas- 

 topore They join BALFOUR when he writes (1881, p. 238): 

 "The primitive streak represents the linear streak connecting 

 the Elasmobranch embryo with the edge of the blastoderm 

 after it has become removed from its previous peripheral posi- 

 tion, as well as the true neurenteric part of the Elasmobranch 

 blastopore". Wrongly enough the adherents of the doctrine 

 of concrescence sometimes compare to the pi imitive streak 

 the concrescence-seam assumed by them in front of the 

 blastopore. To me it seems that one ought to add that a 

 primitive streak is to be expected only in yolk-laden eggs 



