52 
HAROLD SIDEBOTHAM. 
Miss Johnson and Miss Sheldon, writing with reference to 
the Newt, 1 incline to the conclusion that the tail, as well as 
the post-anal gut, is a secondary structure developed after the 
permanent anus. Of course this view would be equally ap- 
plicable to the same structures in the Frog, if Professor 
Spencer is correct as to the fate of the blastopore. 
But I think that the condition which obtains in the Frog at 
the stage when the blastopore is still just open represents 
an extremely primitive condition, for Balfour, in his paragraph 
with reference to the post-anal gut and neurenteric canal, 2 
comes to the conclusion that the neural and alimentary canals 
must have had a common opening, probably into a dilated 
vesicle, before going directly to the exterior. This is exactly 
what is found in the Frog just before the rectal diverticulum 
becomes perforate. 
Professor Spencer only figures one longitudinal section, and 
this is from a stage subsequent to that in which the closure of 
the neurenteric canal takes place, a stage, moreover, at which 
all trace of the blastopore has gone. 
During the time the blastopore is open it always runs in a 
line with the mesenteron and opens posteriorly, while the rectal 
diverticulum always runs ventrally, and keeps at about the 
same angle to the mesenteron, even after the tail has become 
fairly well developed. 
In Professor Spencer’s fig. 15 the canal, the extremity of 
which he marks (an.) and describes as the blastopore, I think 
must be the rectal diverticulum, as I find just the same condi- 
tion as there represented in my sections taken from embryos 
at a corresponding age. 
His figs. 5 and 6 are taken from an earlier stage than his 
fig. 15, and show just the same features as do mine, but the 
appearances they present may easily be interpreted in accord- 
ance with my view, the opening he marks bl. in both figures being 
the proctodacal invagination. In his fig. 5 the blastopore is 
1 Loc. cit. 
' 2 * Comp. Embryology,’ vol. ii, chap. xii. 
