William A. Hilton 485 



Summary. 



1. Crypts of Lieberkiihn in the white rat develop as simple down- 

 growths of epithelium without any relation to the villi. 



2. Crypts begin to develop after the villi are formed and make their 

 appearance first in the rectum. 



3. Villi of the large intestine slowly decrease in size and disappear as 

 the area of the intestine increases. The rectum is found to be without 

 villi first, next the caecum and last the upper large intestine. 



Some Geneeal Conclusions. 



As with other organs, folds and villi in all forms present individual 

 variations, but although great variation is encountered with all species, 

 it is especially noticeable in the lower forms; to take specific examples, 

 the folds and villi of Amia or the folds of the toad vary more among 

 themselves than the villi of calf or cat. . 



Another point observed upon other organs is the recurrence of simi- 

 lar structures in widely differing groups, as illustrated by Amia and 

 Bufo. These have very similar conditions of arrangement of the folds. 

 Other examples would be the similarity between the folds of the black- 

 snake and those of some birds: and the similar folds of catfish and some 

 Amphibia. These widely separated forms which have similar structures 

 may have come to such conditions, either by similarity of environment, 

 that is character of the food; or these types may simply indicate that all 

 forms develop similarly and that several widely differing forms may 

 have developed faster and reached a more specialized condition than 

 their near relatives; or that other forms were for several causes retarded 

 in development, and so approximated the less advanced conditions of 

 less specialized animals. 



The largest villi were usually found in the larger animals, especially 

 when the species were nearly related. When two or more adult animals 

 of the same species were examined, it was found that the larger ani- 

 mals in most cases had larger villi than the smaller ones, and when 

 young and old animals were compared it was found that these differ- 

 ences were much more striking. 



ters, which usually run transversely, are grooves which connect simply the mouths of 

 glands. These occur in the rectum, and in sections the appearance is as if we had 

 crypts with villi overhead and all gradations of villi uniting with crypts and so dis- 

 appearing. This appearance is easily explained after the study of the surface of the 

 intestine. These depressions are of late origin and are only found long after villi 

 have disappeared. 



