William A. Hilton 473 



development this network becomes broken up into a more uneven 

 arrangement by growth of the intestine, and the connections of meshes 

 come to be separated; so here and there villi begin to be isolated from 

 the folds and possibly, also, by farther unequal growth, villi come to 

 entirely replace the folds. This method, although undoubtedly occur- 

 ring in some forms, as in Amia where all grades of such development 

 are shown, is probably not so common as the development from zigzag 

 folds. In birds, for instance, it seems to be rather rare. Develop- 

 ment, as shown by embryology, is very necessary to confirm the rather 

 scattered results of the study of adult conditions. 



Development oe Villi in the Chick. 



Plates III, IV and V, Figures 47-58 and 60-70. 



There is a great variation noticeable in the intestines of chick 

 embryos; even those of the same size and same age may differ much 

 in regard to the development of the mucosa. In the following dis- 

 cussion the time of incubation is taken as a criterion of age and only 

 those embryos are described which seem to show typical stages of 

 development. 



In some early embryos different stages of development may be found 

 in different portions of a single intestine, therefore it might seem 

 ■possible to obtain all stages from one intestine, but this is only true 

 to a certain extent; so specimens of different ages are necessary in 

 order to determine exactly what takes place in all parts of the intestine. 



All parts of a seven day chick's intestine appear without folds or 

 other elevations of the mucosa. 



At eight days' incubation, rather large longitudinal folds of the 

 mucosa make their appearance in the pyloric part of the small intes- 

 tine. A cross section of the intestine at this place usually shows about 

 three large folds (Fig. 48). These folds continue for some distance 

 (Figs. 60, 61), but as the caudal portion of the intestine is approached 

 they decrease in size and number and the last of the intestine shows 

 no folds (Fig. 47) ; or the typical condition of earlier embryos. 



In the small intestine of a nine-day chick there are seen to be six 

 to seven folds which are somewhat smaller than those of the eight-day 

 chick. They are also much more distinct. These folds are parallel, 

 longitudinal and nearly straight (Fig. 49, A and B, and Fig. 62)! 

 Towards the c^cum these folds are less numerous and in the last part 

 of the small intestine they are entirely absent. 



