190 FRANKLIN PARADISE JOHNSON 



measures but 0.04 mm. in diameter. As the urogenital sinus 

 is approached the epithehal tube again increases in size and 

 becomes compressed from side to side. This enlargement passes 

 insensibly over into the epithelium of the cloaca. 



The walls of the epithelial tube also vary in thickness. Above, 

 where it is broadest, the walls have a thickness of about 0.028 

 mm. and are composed of cells which have no distinct cell boun- 

 daries. Three rows of oval nuclei are discernible. In the nar- 

 rower middle portion the walls average about 0.017 mm. in 

 thickness, and show only two rows of nuclei. Where the tube 

 is expanded in the cloacal region, the epithelium is approximately 

 of the same thickness as in the colic ampulla. The lumen in 

 most places is a narrow slit-like cleft, larger in the extremities 

 than in the mid-region of the large intestine, but everywhere 

 present and patent. It communicates with the larger lumen 

 of the cloaca, but cannot be traced through this to the outside 

 because of the presence of the cloacal membrane. 



In an embryo of 10 mm. the large intestine presents practi- 

 cally the same relations, but shows a marked increase in size. 

 The colic ampulla, which is now situated well out in the coelom 

 of the umbihcal cord, measures alout 0.33 mm. in diameter, 

 and has an epithelium 0.045 mm. in thickness. A slight bud- 

 like protuberance of almost the same size as the swelling itself, 

 arises from it, and extends into the mesenchyma. Followed 

 caudally, the epithelial tube of the large intestine quickly dimin- 

 ishes in size, and continues of small size until the region of the 

 rectum is reached. Here it presents another spindle-shaped 

 swelling. This sweUing is connected with the cloaca by a short 

 and narrow tube. The upper narrow portion of the epithelial 

 tube measures about 0.07 mm. in diameter, and its wall is 

 0.028 mm. thick. The sweUing is 0.12 mm. in diameter, and 

 has a wall thickness of 0.036 mm. The lumen is continuous 

 as far as the cloacal membrane where it is closed off from the 

 exterior. 



At 16 mm. the colic ampulla is similar in form but larger than 

 in the preceding stages, being now 0.17 mm. in diameter. It 

 is directly continued into the vermiform process, which is found 



