DEVELOPMENT OF THE LARGE INTESTINE 197 



epithelial tube averages 0.38 mm. The descending colon (fig. 

 18) has a diameter of 0.34 mm., is more rounded in shape, being 

 quite similar to the more cephalic part of the ascending colon. 

 The crests of the folds and ridges are, however, not so angular. 



First appearance of villi 



In the rectum the epithelial folds have increased in size and 

 give to the lumen a very irregular form. As shown in figure 

 19, some of the folds run almost transversely. The presence 

 of transverse folds have been noted in the lower portion of the 

 rectum in a number of older embryos as well. Besides folds, here 

 and there are present conical-shaped projections of the epithelium. 

 These represent the first transitory villi of the large intestine. 



In a number of places the folds seem to be fused together 

 at their tops, shutting off small rounded spaces. These spaces 

 I have determined from serial sections to be epithelial cysts. 

 They are found in corresponding portions of the rectum of other 

 embryos, but are confined to this region of the large intestine 

 alone. A portion of one of these cysts is shown in figure 19 

 at X. They are described in detail below. 



At this point it seems advisable to make the following sum- 

 mary regarding the development of ridges and folds. In the 

 beginning the epithelial tube is cylindrical in shape. The first 

 changes that take place in its form are found in the rectum, 

 where it shows a number of longitudinal ridges. These ridges 

 are the forerunners of folds, for everywhere they later appear 

 as if pushed in from behind by the underlying mesenchyma. 

 Soon afterward ridges and folds are found in the descending 

 colon, the direction of growth being from below upward. How- 

 ever, before these changes have extended into the transverse 

 colon, similar changes are found to be occurring in the ascend- 

 ing colon near to the colic valve. The direction of growth here 

 is opposite that in the descending colon, that is, ab-orally. The 

 transverse colon is, therefore, the last portion of the large intes- 

 tine to develop folds. Similarly, in a few of the subsequent 

 stages, the transverse colon shows a slight retardation in the 



