Mucous Membrane in the Human Embryo. 545 
part of the intestine. As villi are developed they again appear in 
the upper part of the intestine first, so stages are found with villi 
in the duodenum, and only folds in the ileum. Furthermore, 
when villi alone are present, they are more numerous in the duo- 
denum than lower down. The most striking result in the follow- 
ing out the development of the villi in the human intestine is that 
they repeat all the stages found from the standpoint of compara- 
tive anatomy.” 
He concludes, ‘‘ The villiappear first aslongitudinal folds. These 
folds grow larger and then break up into villi.”’ 
Forssner (’07) in a study of cesophageal and intestinal atresiae, 
while not mainly concerned with the development of villi, con- 
firms the work of Berry in that the villi of the human small intes- 
tine appear first as longitudinal folds. 
In the human embryos I have studied, only in the lower part of 
the small intestine was I able to find distinct longitudinal folds 
preceding the formation of villi. In the upper two thirds of the 
small intestine, villi are found developing as knob-like invagina- 
tions of the epithelium. 
The first evidences of villi found are in an embryo of 19 mm. 
Immediately below che pyloric end of the stomach, the duodenal 
epithelium shows, besides its vacuoles, other irregularities. These 
are thickenings of the epithelium and invaginations of the epithe- 
lium into the lumen of the intestine, and represent different 
stages in the development of villi. The thickenings vary in size 
and height, some being twice as thick as the epithelium of the in- 
testine in other places. The invaginations are slight rounded 
elevations of epithelium, and have developed from the thickenings 
by a pushing in of the mesenchyma, which now forms the cores 
of these small villi. That these are villi and not folds is evident 
because they can be followed through only four to six 23 » sections 
and in breadth at their bases they measure from .10 to .14 mm. 
From these measurements the bases of these elevations are seen 
to be approximately circular in cross section. Villi are of all 
gradations in height, one of the tallest measuring .09 mm. (The 
height of a villus has in every case been measured on the mesen- 
chymal surface, v.e., the depth of the hole which is filled with mes- 
