or 
74 ALICE JOHNSON AND LILIAN SHELDON. 
and its relations there, as described below, explain this diffi- 
culty. In a transverse section taken a very short distance in 
front of the blastopore (anus), a portion of the dorsal wall of 
the gut is partially constricted off (fig. 1), and a little further 
back becomes completely separate (figs. 2 and 3), and may be 
traced back into the tail as a solid mass of cells, lying just 
below the notochord. Near the posterior end of the tail this 
mass dilates (fig. 5), forming a portion which is probably 
homologous with the caudal vesicle of the post-anal gut in 
Elasmobranchs (1), and then fuses with the other structures in 
the tail at the extreme end (figs. 6, 7). 
This solid diverticulum of the alimentary canal appears from 
its relations to be the post-anal gut, and its point of fusion 
with the notochord and neural canal no doubt represents the 
neurenteric canal. 
At earlier stages the ucurenteric canal, which we believe to 
be always solid in the Newt, though open for a short time in 
the Frog, is represented by the point at which the fused layers 
pass into the blastopore. The neurenteric canal is then, 
roughly speaking, vertical in direction, since the blastopore is 
situated at the hind end of the ventral surface. When the tail 
grows out behind the blastopore, the middle point of the 
vertical neurenteric canal grows out with it, remaining always 
at its tip, so that the canal becomes, as it were, drawn out into 
a loop with dorsal and ventral horizontal limbs. The tail is at 
first composed of undifferentiated tissue, and the differentiation 
proceeds as usual from before backwards, the dorsal limb of 
the loop being the medullary canal, and the ventral the post- 
anal gut. The two limbs are still connected at the posterior 
end of the tail by the neurenteric canal. 
This mode of development seems to us to show that the tail 
with the post-anal gut is a secondary structure, developed 
after the permanent anus. The function of the post-anal gut 
seems to be to provide material for the growth of the tail 
during embryonic stages before the blood-vessels have formed. 
With the appearance of the latter, the post-anal gut gradually 
atrophies, a remnant of it being attached to the rectum just in 
