DEVELOPMENT OF THE LARGE INTESTINE 199 



rows, suggesting that the folds have become broken up into 

 segments. The folds and villi measure from 0.08 to 0.11 mm. 

 in height and are usually between 0.07 and 0.11 mm. in width 

 at their bases. The epithelium on their tops is distinctly sim- 

 ple columnar in form, and is reduced to 0.019 mm. in thickness; 

 between the folds it is 0.031 mm. thick. 



Numerous villi are found in the ascending colon. A compari- 

 son of these with those villi in the lower part of the ileum shows 

 that the two are quite similar in form and size. At a short dis- 

 tance from the colic valve, the epithelial walls of the ascending 

 colon become pushed in by three large mesenchymal folds, re- 

 ducing the lumen to a narrow F-shaped cleft. Here are found 

 folds and villi resembling those shown in figure 20. The de- 

 scending colon is smaller in diameter than the transverse, being 

 only 0.45 mm. An examination of its inner surface (fig. 21), 

 shows folds and villi, and what apparently are partially formed 

 villi, about eight to nine rows in all. These are longer than 

 those of the transverse colon, 0.17 to 0.22 mm., but of about 

 the same width. In the sigmoid colon is found a condition 

 comparable to that of the descending colon. However, goblet 

 cells are far more numerous. 



First appearance of intestinal glands 



The epithelial tube of the upper portion of the rectum (embryo 

 of 65 mm.) is flattened from side to side, and measures 1.17 

 by 0.77 mm. in cross section. A very different appearance 

 is presented from that of the transverse colon. The epithelial 

 wall is bent into a number of folds which are closely packed to- 

 gether. Many of these measure as much as 0.27 and 0.36 mm. 

 in height. The bottoms of the spaces between these projections 

 are developing glands. Where they are cut obliquely or in 

 cross section their basal ends are seen to be tubular in form and 

 provided with small round lumina. 



Epithelial cysts are more numerous than in the preceding 

 embryo. They represent glands and intervillous spaces which 

 have become closed over at their tops. They show evidences 



THE AMERICAN JOURNAL OP ANATOMY, VOL. 14, NO. 2 



