478 Intestinal Folds and Villi in Vertebrates 



found to contain a central lacteal surrounded by the usual villus net- 

 work of blood vessels. These little elevations resemble the villi of 

 the duodenum more closely than any of the other villi of rabbit and 

 they are without doubt true villi, although much reduced in size. In 

 some animals, such as raccoon, where no caecum is present, only the 

 lower part of the intestine is free from villi. 



Excluding the forms mentioned above, mammals may be said to 

 have no villi in the csecum or large intestine of the adult. 



Villi are usually present in birds and in the majority of forms 

 where they occur they are found throughout the small and large 

 intestine including the caeca, although in some eases the lower large 

 intestine and tips of the caeca present variations in this respect. 



Although villi do not occur in the large intestines of adult mam- 

 mals, they do occur in the intestines of advanced embryos;* and in 

 some cases where the young are born in a very immature condition, 

 villi persist for some time after birth. The mammals in which villi 

 are found in the large intestine for a short time after birth, have not 

 been determined to any extent, but at least two forms in which this is 

 the case are rabbit and white rat. Examples of intestines where villi 

 disappear before birth are those of man and cat. 



In a consideration of the way in which villi disappear in the large 

 intestine of mammals, the following will be confined to their disap- 

 pearance in the white rat. In this form they persist for a considerable 

 timr3 after the rats are born, and are not entirely absent from the large 

 intestine until 12-14 days after birth. 



Before starting with the discussion of the manner in which villi 

 disappear, it will be necessary to speak of the development of the crypts 

 of Lieberkiihn, for it has been assumed that there is an intimate rela- 

 tion between the development of the crypts of Lieberkiihn and the dis- 

 appearance of villi. 



Lieberklihnen crypts occur in the adult scattered irregularly in the 

 small intestine. They are especially numerous in the ileum, but in 

 the large intestine including the caecum, they are so numerous that 

 their mouths take up a large part of the intestinal surface. These 

 crypts are simple or occasionally two-branched tubes or follicles of 

 epithelium extending down into mucosa and usually nearly reaching to 

 the muscular coats of the intestine. 



4 An interesting point in regard to the intestine of man which has apparently 

 hitherto escaped observation is the fact that villi are found at one time in the vermi- 

 form appendix. 



