472 Intestinal Folds and Villi in Vertebrates 



base line and the relative breadth of villi by the distance of the dotted 

 lines from the base line. The left hand of each diagram represents the 

 cephalic, and the right the caudal end of the intestine. 



In Figs. 1, 2 and 4 of birds, the cseca are indicated by the heavy 

 perpendiculars above and below the base line. All of the base line to 

 the right of these perpendiculars indicates the small intestine. 



In Figs. 1 and 2 large folds in the cgeca are indicated by the curved 

 lines which are the farthest from the base line. 



THE DEVELOPMENT OF FOLDS AND VILLI. 



The development of folds and villi may be considered in two ways: 



1st, by the study of the different stages found in adult species. 



2nd, by the study of their embryological development in one or 

 more animals. 



By a brief review of the typical forms found in some intestines 

 described, it may be seen that in a simple type of intestine only slight 

 longitudinal folds occur. In some other species there are rather more 

 prominent, nearly straight folds; in others, wavy folds are found, and 

 in some others very wavy or zigzag folds may be seen. In a few 

 species, net-arranged folds are observed. 



The investigations of recent years and the researches described in 

 the present paper, point unmistakably to the conclusion that both 

 phylogenetically and ontogenetically villi are developed from mucosal 

 folds. The phylogenetic development may be illustrated in two ways 

 by various groups and may be made out quite well in animals where 

 folds and villi occur in the same intestine. 



The simplest way in which villi are developed from folds is their 

 •formation from long, very zigzag, longitudinal ones, such as described 

 in the blacksnake, sea gull, night heron, cedar bird, etc. Villi seem 

 to be formed by simple separations at the angles of the folds. In sev- 

 eral forms of animals, such as the wild duck or the night heron, zigzag 

 continuous folds are found on one part of the intestine and in other 

 places the villi are arranged in zigzag rows; these rows look like folds 

 but are broken up into villi. In some intestines, such as those of the 

 chicken or the turkey, villi are found which are not regularly arranged 

 and no folds occur in the adult. 



Another less common method of development of villi is the develop- 

 ment from the net-formed folds. In this method probably very zigzag 

 folds unite with each other, making a rather even network; later in 



