126 W. BALDWIN SPENCER. 
which opens into the blastopore, becomes closed though the 
blastopore itself remains open. This apparently is caused by 
growth of the epiblast cells lining the neural canal, and, inas- 
much as they contain much pigment, the position of the former 
canal, connecting the neural and alimentary canals, is plainly 
indicated by a line of dark cells seen well in section. 
Figs. 12—14 represent transverse sections through an embryo 
of the age of fig. 4, in which the primitive blastopore still opens 
into the alimentary canal, but yet no open connection exists 
between this and the neural canal, which posteriorly ends in a 
solid cord of cells. At this period, as is well seen in fig. 15, 
which represents a longitudinal vertical section of an embryo, 
there is, at the posterior end around the blastopore, a fusion of 
the different cell layers. Into this fused mass appear to enter 
the cells of the external epiblast, of the neural canal, of the 
notochord, of the hypoblast of the alimentary canal, and of the 
yolk, whilst at a slight distance anteriorly the mesoblast grows 
out of this. 
Thus both sections and external views lead to the same con- 
clusion, that in Rana, as in Triton, the blastopore is transformed 
into the permanent anus, though there is the not unimportant 
difference between the two, that in Triton no connection exists 
between the neural and alimentary canals, whilst such a struc- 
ture is well marked in Rana. 
Balfour! (after Gotte) figures a longitudinal section of 
Bombinator at a stage considerably after the neural folds have 
met and fused, and when, according to this older theory, the 
blastopore has been enclosed and a neurenteric canal formed. 
The same section shows the formation of a proctodzum, and 
the presence of a so-called post-anal gut. In no section out 
of the numerous series cut have either of these structures been 
seen in Rana, and yet, inasmuch as, by means of dorsal growth, 
the blastopore is carried round the posterior end till it lies 
some little distance anteriorly on the ventral side, there is a 
time at which the hinder end of the neural canal opening into 
the blastopore occupies exactly the position of the post-anal 
1 ©Comp. Embryolog.,’ vol. ii, p. 107. 
