Mucous Membrane in the Human Embryo. 549 
in the narrowed portion, two distinct folds, the tops of which 
are irregular in height, are present. 
In the jejunum at 37 mm., the villi are similar to those at 30 
mm., but are now regular in shape. They appear to be fused to a 
greater extent, and there are more longitudinal rows. In shape 
the villi vary from small low swellings to rounded or elongated 
processes, some of which are .214mm. in height. They are only 
a little taller in the duodenum than below. Further down, at 
the entrance of a persistent yolk stalk, the intestine becomes wide. 
In this widened portion both villi and thickenings are seen. Be- 
low this region the diameter of the epithelial tube becomes smaller 
and irregular longitudinal folds are found. Still further down 
only low ridges are found which have no corresponding depres- 
sions for the mesenchyma. These ridges extend as far as the ileo- 
eaecal valve. Practically the same conditions are found in an em- 
bryo of 42 mm., there being a few folds in the lower part of the 
ileum where the lumen is narrow. Where it is wide, villi are found. 
In the mid region of the small intestine at 55 mm. (fig. 21), 
the villi have greatly increased in size and in number. They are, 
for the most part, rounded conical processes but a few are ir- 
regular and blunted. A most striking fact is that the villi are all 
very near the same size. Since additional villi develop as separate 
invaginations, one would expect to always find a greater number of 
small villi than large ones. But the small villi have always 
been found to be less numerous. It follows, therefore, that their 
growth must be more rapid than that of the older villi. Additional 
rows of villi have developed between those already present. 
At 55 mm. there are about 13 rows in the mid region of the small 
intestine. The villi of the new rows are as tall as those of the older 
ones, in fact, the later developed rows cannot be distinguished 
from the earlier ones. The arrangement of the villi in rows is 
best seen in a view of the external wall of the epithelium (fig. 22). 
In the extreme lower end of the ileum at this stage (55 mm.) 
two or three slight folds are found. 
Regarding the further development of villi, there is but little 
of interest. They gradually increase in number and in size through- 
out stages from 73 mm. to birth (Tables 3 and 4). In the later 
