GASTRULATION AND EARLIEST DEVELOPMENT 195 



elusion that the anterior half of the slit-like blastopore is 

 transformed into the neurenteric canal, the posterior half 

 into the anus. Yet in Pelobates he claims that this posterior 

 half first closes and that the anus forms only later. 



As is apparent from the foregoing, during this period 

 nearly every year brought forth a new investigation on 

 this subject. In 1890 that of Erlanger on Rana esculenta 

 appeared; in 1891 that of ROBINSON and ASSHETON on 

 Rana temporaria ; in the same year a small treatise by 

 Erlanger in reply to some observations made by the 

 two English authors on his work All three authors agree, 

 howevei, that in both cases the anus arises by perforation. 



In later years the fate of the blastopore is alluded to 

 only in a few investigations, eg by Bles (1905) who 

 for Xenopus laevis, and by SEEM ANN (1907) who for 

 Alytes obstetricans finds that the blastopore is not enclosed 

 by the medullary folds and passes directly irto the anus, 

 there being accordingly no neurenteric canal. 



Rana esculenta — Most of the investigators who have 

 paid special attention to the question thus come to the 

 conclusion (which after my own examination of Rana 

 esculenta I can support without reservation) that the anus 

 arises by perforation a little distance behind the blastopore 

 which is transformed into the neurenteric canal. A short 

 description may be given here in addition to the figures 

 for Rana esculenta. 



After the yolk-plug has disappeared from the surface 

 the blastopore presents itself as a short longitudinal slit 

 (fig. 40 a) A median section through this egg is repro- 

 duced in fig. I of the plate II. In a similar longitudinal 

 series one succeeds better than might be expected in getting 

 the blastopore as an opening (bl), though of course this 

 is only the c^se in one or two sections. The ventral blasto- 

 pore lip is well developed and includes between itself 

 and the yolk cell mass in the archenteron the so-called 

 anal diverticulum (Afterdarm, a. d.) which, however, is 

 nothing but the intersection of the circular incision sur- 

 rounding the mass of yolk-cells. 



In a somewhat advanced stage a shallow impression in 

 the ectoderm {a) appears on the surface of the egg (fig. 40 

 b) behind the slit-like blastopore. This impression is clearly 

 visible in a longitudinal section, as in fig. 2 of the plate. 

 Underneath this impression a thickening of the ectoderm 



