William A. Hilton 477 



No wavy or zigzag folds were found in any of the specimens but in 

 every case, villi either developed from parallel or irregular folds (Figs. 

 73, 74 and 79-80), or arose entirely without folds. 



An embryo of 34 mm. which was much more advanced than the one 

 of 33 mm., showed the lower parts of the intestine more or less free 

 from folds, but in most of the intestine various stages of folds were 

 found partly broken up into villi (Fig. 73). In some cases villi appear 

 to be formed without passing through a fold stage, but usually, as in 

 different parts of the intestine of this embryo, villi were formed from 

 folds, that is, these folds grew higher and became divided into smaller 

 but higher portions by irregular and unequal growth, and these pro- 

 jections soon grew like the villi of the adult. 



The large intestine of the white rat is rather backward in its devel- 

 opment, villi are not formed until the embryo is 40-50 mm. in length 

 and then as in chick, the villi first appear as solid epithelial processes 

 into which later the connective-tissue cores extend. Such a develop- 

 ment of villi has been described in the large intestine by Brand, 77, 

 and Patzelt, 82. 



In the white rat the development of villi may be summarized as 

 follows : 



Folds are first formed near the pylorus ; they are rather large, regular 

 and parallel. Similar folds develop later in other parts of the intes- 

 tine, but in some places no folds are formed. By unequal growths the 

 folds gradually break up into villi and new villi are also developed 

 without passing through a fold stage (Fig. 57). The beginnings of 

 villi in the large intestine may be first recognized as little elevations 

 of epithelial cells (Fig. 59). A core of connective tissue pushes up 

 into these cell masses and the villi grow upward and become like those 

 in other parts of the intestine. 



The development of villi seems to take place rather early as com- 

 pared with chick and the zigzag fold stage so characteristic of birds is 

 lacking. 



DiSAPPEAEAISrCE OF ViLLI PEOM THE LaEGE INTESTINES OF 



Mammals. 



(Figures 81-87.) 



It is well known that villi occur imiformly in the small intestine 

 alone in adult mammals, but there are a few exceptions. In the cgecum 

 of adult rabbits there are a number of very low mounds which are 



