FATE OF TEE BLASTOPORE IN RANA TEMPORARIA. 51 
Fig. 2 shows the next stage in an embryo with five meso- 
blastic somites. The neural folds have met and form a tube, 
bending over the posterior end of the embryo, and opening 
through the dorsal region of the blastopore, which has become 
narrower and longer. Below the blastopore the proctodaeal 
invagination may be seen to have increased considerably in 
depth as also has the rectal diverticulum from the mesenterou. 
In fig. 3 the same parts can be recognised, but the closure of 
the blastopore has proceeded further. This stage has six meso- 
blastic somites. The septum dividing the rectal diverticulum 
from the proctodaeal invagination has become perforate. The 
section of which fig. 3 is a drawing is not quite in the right 
plane for showing the neurenteric canal. 
Fig. 4 shows the next stage in an embryo with eight meso- 
blastic somites. The blastopore is completely occluded, and is 
represented by a column of epiblastic cells ( ep .) which may 
be recognised by their containing much pigment. The anus 
communicates freely with the alimentary canal, and the 
neurenteric canal is still well marked. There is a slight 
dilatation of this canal at the junction of its dorsal and ventral 
limbs, which dilatation is the post-anal vesicle. 
In the next stage, with nine mesoblastic somites, represented 
by fig. 5, the tail has just commenced to bud out. The rem- 
nant of the blastopore is represented only by a little heaping 
together of the epiblast cells, where the blastopore opened to 
the surface. The neurenteric canal has become occluded, but 
still can be traced up as a solid rod of cells, representing the 
post-anal gut, commencing from the normal position. There 
is a diverticulum from the proctodseum just before it opens to 
the surface which is the rudiment of the allantoic bladder. 
Thus the history differs from that given by Balfour, in that 
the neural folds do not enclose the blastopore, the closure of 
the blastopore being effected subsequently to the meeting of 
the neural folds. My conclusion differs essentially from the 
description given by Professor Spencer, inasmuch as I find 
that the anus is not derived from a persistent blastopore, but 
is formed from an independent proctodmal invagination. 
