THE LATER DEVELOPMENT OF THE FROG 169 



the body cavity from the posterior end of the stomach (Fig. 59). 

 The elongation of the intestine continues rapidly, and soon it 

 becomes thrown into a double spiral which occupies the entire 

 ventral part of the body cavity. At its maximum length it is 

 about nine times the length of the body. The oesophagus 

 and stomach also elongate somewhat during this period, so 

 that the pancreas and liver are pushed back into the body 

 cavity. The relations of the mesentery are described below. 

 During the period of metamorphosis the entire digestive tract 

 shortens to about one-third its maximum larval length; this 

 shortening affects chiefly the intestine and stomach. 



One structure developing in connection with the enteron 

 has not been mentioned as yet; this is the hypochordal rod. 

 This has no direct relation with the notochord. It appears 

 in tadpoles of about eight somites (3-4 mm.) as a median 

 ridge along the outer surface of the endodermal wall of the mid- 

 gut (Fig. 70, C). Later this ridge extends both anteriorly 

 and posteriorly, as the part first formed separates from the 

 enteric wall; it becomes entirely free at about 4.6 mm. Finally 

 it extends the entire length of the gut posterior to the dorsal 

 pancreas; through the tail it lies above the postanal gut. It 

 is a narrow rod, only two or three cells in diameter, lying be- 

 tween the dorsal aorta and the notochord. Shortly after 

 the opening of the mouth (13 mm.) it breaks into short pieces 

 and its cells either disappear or scatter; in the older larva no 

 traces remain. 



3. The Derivatives of the Hind-gut 



This is the smallest section of the enteron. We have de- 

 scribed, in the preceding chapter the formation of the neuren- 

 teric canal and proctodaBum, and the terminal dilation of the 

 enteron which becomes the rectal region (Fig. 37). Just after 

 the tail has begun to elongate (4 mm.) the fusion between the 

 rectal and proctodaBal walls becomes perforated by the anal 

 opening, so that the gut opens directly to the outside. The 

 proctocteal region becomes the cloaca of the tadpole and frog, 



