474 Intestinal Folds and Villi in Vertebrates 



In the duodenum of a ten-day chick, there are from 9-16 longitudinal 

 folds which vary somewhat in size. Some are low and rather shorter 

 than others. Most of the folds near the pylorus are rather wavy from 

 side to side, but some are nearly straight. Farther along in the small 

 intestine the folds are less numerous or entirely absent. 



In the small intestine of an eleven-day chick the longitudinal folds 

 near the pylorus have become quite wavy from side to side (Fig. 50 

 A and B, and Figs. 63-64). Farther towards the caeca they are less 

 numerous and not so wavy; still farther along there are only about 

 ten straight folds; and next the caeca there are a few folds or mound- 

 like elevations, shorter and thicker than the others. 



In the small intestine of a thirteen-day chick the folds are found to 

 be very zigzag, the angles made by the folds are quite sharp; the tops 

 of these folds are more or less divided, that is, partially broken up into 

 villi (Figs. 51, 52). Lower in the intestine, similar folds are observed 

 for some distance, or in places irregular elevations of epithelium which 

 are possibly short folds. Towards the caeca the folds nearly disappear. 



In a chick of fourteen days almost perfectly formed villi are found 

 in the duodenum (Fig. 53). In sagittal section these villi are seen to 

 be united at their bases, so they form zigzag folds, partly divided into 

 villi at the free edges (Fig. 54). Villi have been formed in part by 

 folds becoming very zigzag, the epithelium of the sharp fold angles is 

 in this way gradually brought in contact and then separation into villi 

 takes place from the tips downward (Figs. 69 and 70). 



Lower in the small intestine less advanced conditions of folds are 

 found, but just above the caeca the conditions noticed cephalad are 

 much less evident and it is very probable that villi are formed here, 

 without going through all the fold stages. 



In a fifteen-day chick there was found a remnant of the zigzag 

 arrangement of villi; short folds were noticed similar to the larger 

 ones of earlier embryos. 



In the small intestine of a sixteen-day chick there are true villi and 

 the zigzag arrangement is absent throughout. 



Up to about ten days of incubation there are no folds in the CEBca, 

 but at ten days a few rather large irregular folds appear. At eleven 

 days a few large thick elevations may be seen, but most of each caecum 

 is bare of folds of any sort. At thirteen days the caeca have thick 

 irregular mounds (Figs. 55 and 67). These elevations grow upward 

 irregularly so that some parts outstrip others; as growth proceeds, the 

 elevations become divided into smaller and higher projections and at 

 fourteen days these may be spoken of as low villi of various sizes. 



