146 
clusion 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, 
however, 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 SEEMANN (1907) who for 
Alytes obstetricans finds that the blastopore is not enclosed 
by the medullary folds and passes directly into 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 1 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. Î of the plate Il. 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 case 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. hen 
In a somewhat advanced stage a shallow impresion 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 
