476 Intestinal Folds and Villi in Vertebrates 



In most cases the large intestine remains without villi for a longer 

 period than the caeca; here the thick irregular elevations similar to 

 those in the caeca are seldom found. In the large intestine numerous 

 very small processes composed entirely of epithelial cells make their 

 appearance, connective-tissue cores penetrate into these projections and 

 in this way villi are formed without passing through a fold stage. 

 Later villi develop as simple upgrowths of the mucosa, probably with- 

 out being preceded by the small, purely epithelial processes. 



The Developitent of Villi in Mammals. 



Plate VI and Fig. 59. 



The development of villi in mammals has been recently studied by 

 Voigt, 99, and Berry, 1900. Voigt speaks of villi formed in pig from 

 large elevations of the mucosa which gradually become broken up by 

 means of depressions; from these elevations villi grow up. Berry de- 

 scribes a similar method of villus formation in man, but he shows a 

 little more clearly that the first elevations of the mucosa are in the 

 form of rather large, irregular, longitudinal folds; these folds become 

 broken up into villi. 



The following presentation of the development of villi in mammals is 

 confined to the intestine of the white rat. 



As no idea could be obtained in regard to the age of the embryos, size 

 alone is given, although this often expresses very little about the stage 

 of the embryo's development. 



Nowhere near the amount of material was available as in the study 

 of chick, but in a way it was not required because the development of 

 the villi is more direct and uniform than in chick. 



The first indications of folds in the intestine of the white rat was 

 seen in embryos of about 16-20 mm. from the base of the tail to the tip 

 of the snout. The first folds are usually made up of very thick epithe- 

 lial masses, these are somewhat short and generally run parallel, with 

 the lumen of the intestine (Fig. 75). Into some of these very early 

 folds a developing core of connective tissue may be seen. In an em- 

 bryo of 33 mm. there were several large, thick, rather regular and 

 parallel folds with well-marked connective-tissue cores ; there were four 

 near the pylorus which nearly filled the lumen (Figs. 76 and 77), two 

 were large and two smaller, but after extending parallel for about 1.5 

 mm. there were only three folds and 1 mm. farther caudad only two 

 folds and soon these ended (Figs. 71 and 72). 



