242 ALICE JOHNSON AND LILIAN SHELDON. 



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 ofif (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 cellSj 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 

 ueurenteric canal. 



At earlier stages the ueurenteric 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 ueurenteric 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 ueurenteric 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 difi'erentiation 

 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 neur enteric 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 

 front of the anus in a newly hatched larva. At this time it is 



