484 Intestinal Folds and Villi in Vertebrates 



run slantingly about the lumen. These folds of mucosa begin some time 

 before birth and grow larger until in the adult they are of considerable 

 size. They are at first largely made up of epithelium with four or more 

 irregular layers of nucloi. These epithelial folds at first contain a small 

 core of connective tissue, but this soon grows larger and the epithelium 

 becomes similar to that of the adult. Glands grow into the core of these 

 folds until in the adult these folds are almost solid masses of glands. 

 These folds radiate from the junction of the caecum and colon and from 

 one or more thickened centres of intestinal mucous membrane. They 

 are without doubt homologous with the papillge filled with Lieberkiihnen 

 crypts which occur in a similar location in the rabbit's intestine. As to 

 the bearing these folds have upon the disappearance of -villi, it seems 

 probable that the growth of these folds inward must have kept the 

 mucous membrane of this region in a more folded condition than else- 

 where, and it may be that villi were not so quickly drawn out as in other 

 places. As soon as a villus begins to become of less height, the reverse 

 of the process which takes place in the growth of late villi, it often hap- 

 pens that a crypt starts to develop very near it. Perhaps the growth 

 of this crypt downward may have some influence on the disappearance 

 of the villus, but crypts do not grow down near every villus which is 

 beginning to disappear (Fig. 82). 



A few of the reasons for connecting the disappearance of villi from 

 the large intestine with the increase in extent of intestinal surface are: 



1. The rapid growth of the intestine; the lack of growth of the villi; 

 and the same size of the glands afterwards as before. 



2. The glands and villi are independent structures; the glands de- 

 velop as downgrowths, the villi as upgrowths. 



3. Villi of gradually less height are formed as the surface of the intes- 

 tine increases in extent and in many cases villi may disappear where no 

 glands are near, or a gland or glands on only a small part of the villus 

 base; some of these glands may start in the base of a villus after it 

 is reduced to a small projection (Fig. 27). 



The erroneous idea that villi disappear by growing together is very 

 easy to arrive at because in later stages the crypts of the large intestine 

 become so numerous that in sections there is much the appearance of 

 numerous short villi, which are, of course, small portions of mucosa be- 

 tween crypts." 



6 In the lower large intestine of a white rat of four days after birth, there some- 

 times occur, besides crypts, gutters which on surface views seem to connect the 

 crypts with each other, but on more careful examination it is found that these gut- 



