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canal is not continued any further between them, as 
MORGAN has already remarked. Hence the opinon of many 
investigators that the medullary folds do not reach to the 
blastopore and that there ís no neurenteric canal. The 
hindmost part of the neurenteric pore remains open as 
the internal opening of the anus. The result is really that 
the hindwall of the hindmost part of the medullary tube is 
perforated by the anus, which in Anurans arises directly 
behind it, and this ís 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), 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 longitudirial 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 post- 
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 primitive streak 
the concrescence-seam assumed by them in front of the 
blastopore. To me it seems that one ought to add that & 
primitive streak is to be expected only in yolk-laden eggs 
