William A. Hilton 475 



thickly placed and irregularly distributed (Figs. 56 and <i6). The 

 further growth and multiplication of villi in the caeca was found to 

 be the same as in other parts of the intestine. The development of 

 villi in the large intestine does not begin until eleven or twelve days 

 of incubation, then small folds occur in some places, these may go on 

 growing and help to form villi in the way in which they are formed in 

 the CEeca, but later stages show very little signs of villi formed as they 

 are in the casca. Thirteen- or fourteen-day chicks show quite uni- 

 formly throughout the large intestine, very small elevations which 

 begin at first as simple elevations of epithelium, later a connective- 

 tissue core seems to push up into these little elevations and true villi 

 are formed by further growth (Figs. 58 and 65). 



The development of villi in chick may be summarized as follows : 

 At first the intestine is entirely free from elevations of any sort. 

 Later, rather large parallel folds of the mucosa gradually make their 

 appearance in the small intestine; first x^ear the pylorus. These folds 

 soon become more numerous, the large folds divide into smaller ones 

 and rather small new folds are developed between those originally 

 formed; and in some cases these later folds also pass through all the 

 stages which the earlier ones do. The slightly wavy folds become more 

 wavy, first in the pyloric part of the intestine, and later more caudad; 

 this wavy appearance becomes more and more marked until those which 

 are at first slightly zigzag grow very zigzag. The angles become very 

 acute and epithelial cells from both sides of many of these begin to 

 nearly touch each other in the centre of the folds. This last is espe- 

 cially true of the tips of the folds. Villi are later formed by unequal 

 growth and separations at the sharp fold angles from the tips down- 

 ward. This separation begins when the folds are first very zigzag; 

 the complete separation into villi occurs quite late in some parts of 

 the small intestine, and long after the first folds are developed, villi 

 make their appearance between other villi, without passing through 

 a fold stage. In the lower part of the small intestine, many of the 

 stages noticed in the upper intestine are entirely omitted and villi are 

 developed, either from short irregular elevations, or as simple upgrowths 

 of the mucosa. 



After all traces of folds are lost the villi grow very rapidly in height 

 and new villi are formed as simple upgrowths of the mucosa. Villi in 

 the casca do not make their appearance until some time after their 

 beginnings are formed in the small intestine. They are formed from 

 thick irregular elevations or short mucosal folds which gradually be- 

 come divided into smaller but higher projections by their own unequal 

 growth. 



