148 
In a slightly further advanced stage (fig. 40d) the greater 
part of the slit-like blastopore has been overgrown by the 
medullary folds, only at the hindmost extremity is there 
still a little opening (bl) from which the anal groove runs to 
the anal pit (a). This anal groove, with a deeper impression 
at its anterior (rest blastopore) and at its posterior end 
(anal pit) appears to have been confused by several authors 
with the slit-like blastopore of a somewhat earlier stage 
(fig. 40 a and 6). They accordingly imagine this slit to 
have closed in the middle by coalescence of the opposite 
borders, leaving only a passage at the anterior and at the 
rear end, these being the future neurenteric canal and the 
anus. The rudiment of the tail is claimed to arise as a 
double knob at the right and the left side of the line of 
coalescence, these knobs fusing afterwards over the middle 
of the blastopore. Thus ZIEGLER (1892), in his note on 
the surface views of Rana-embryos, writes: “Etwas später 
sieht man an Stelle des Spaltes eine Rinne, welche vorn 
in den Canalis neurentericus, hinten in die Aftergrube 
übergeht; es sind nämlich jetzt die seitlichen Blastoporus- 
lippen median zur Vereinigung gekommen”. The same 
views are put forward by HERTWIG in his Lehrbuch. Even 
a close examination of surface views, however, teaches 
us that the anal groove is by no means identical with the 
slit-like blastopore but that its anterior end coincides with 
the rear end of the latter. In fig. 40 c we see it already 
running from the slit-like blastopore to the anal groove. 
The study of median sections precludes every possibility 
of doubt. These sections invariably give the condition of 
fig. 4 (plate) which links up with fig. 3. The anus Is On 
the point of breaking through, the blastopore of being 
closed by the medullary folds. The two are quite 
independent. d 
The step to fig. 5 (plate II) is again a small one. In fig. 3 
we see the cerebral plate already curving in. Especially notic- 
able is the opposition between the praechordal cerebral 
plate and the epichordal medullary plate which as a matter of 
fact is in this stage no longer a flat plate but curved into 4 
groove between the medullaiy folds. Fig. 3, therefore, IS realí- 
zed only in one or two sections which are exactly median; to 
the right or the left side immediately one of the medullary 
folds is intersected, as indicated in fig. 3 with a dotted 
line. A paramedian section in this series thus offers a much 
