590 THE EMBRYOLOGY OF THE FROG 



transverse loop called the duodenal loop, with its proximal end at the 

 posterior portion of the stomach. In fact, the intestine grows to almost 

 nine times the length of the tadpole's body, but is shortened later to 

 about one-third the body-length by the time metamorphosis takes place. 

 The Hypochordal rod is a structure which forms from the mid-gut 

 as a medial ridge along the surface of its entodermal wall just under- 

 neath the notochord, although it has no relation to the notochord. It ap- 

 pears when the tadpole is three or four millimeters in length, and ex- 

 tends both cephalad and caudad, coming to lie free of the gut wall when 

 the tadpole reaches a length of about 4.6 millimeters. Finally, it can 

 be seen as a caudal extension from the dorsal pancreas posterior through 

 the tail. The rod itself is narrow, being only two or three cells in diam- 

 eter. Just before the opening of the mouth, it breaks into pieces and 

 disappears entirely. 



THE DERIVATIVES OF THE HIND-GUT 



It is in this region that the neurenteric canal is found .(Fig. 328), 

 as well as the proctodaeum. The terminal end of the hind-gut, which 

 is to become the rectum, fuses with the proctodaeum, and the anal open- 

 ing perforates the thin sheet which has separated these two structures. 

 This fusion and perforation takes place at about the time the tail begins 

 to elongate, namely, when the embryo is about four millimeters in length. 

 It is the proctodaeal region of both tadpole and frog which forms the 

 cloaca into which rectal, excretory, and reproductive ducts enter. The 

 Bladder forms as a ventral outgrowth from the cloaca just before meta- 

 morphosis. 



The Post-Anal gut is formed by the true hind-gut remaining within 

 the embryo body proper as the tail continues growing. The nerve cord 

 and notochord, however, are carried along in the growing tail so that 

 the neurenteric canal is drawn out caudad. This neurenteric canal is 

 then cut off from the nerve cord, but for a short period its antero-ven- 

 tral portion opens into the rectum and it is this which is known as the 

 post-anal gut. This gradually closes, although a strand of cells can still 

 be seen in the region at the time of hatching, extending nearly to the tip 

 of the tail. It finally disappears altogether. 



