Mucous Membrane in the Human Embryo. 553 
as described by Barth, but may or may not be forked at their 
origins. They can be distinguished from the intestinal glands by 
the clearer protoplasm of their cells, by their branching, and by the 
greater distance that they extend into the mesenchyma. <As the 
muscularis mucosae has not developed, there is as yet no line of 
division between the mucosa and the submucosa. 
In the middle third of the duodenum at 91 mm., no duodenal 
glands are present. At 120 mm. they are present in the whole 
of the duodenum. They aremorenumerousin the upper third than 
lower down, and branching has gone on to a greater extent. In 
the upper third of the duodenum at 187 mm. the glands are much 
larger and the branches of each gland are arranged in groups. 
At the base, toward the muscularis, there is a slight condensa- 
tion of the mesenchyma, which appears to have been caused by 
the downward growth of the gland. Only a few glands are found 
in the middle third of the duodenum at this stage (187 mm.). 
Fig. 24, from a wax reconstruction, shows the structure of the 
duodenal glands at 240 mm. The intestinal glands are the short 
tubular ones arising from around the bases of the villi; the duo- 
denal glands are larger and more branched. The fewness of the 
primary sprouts in the area modelled is striking, there being only 
four for the entire number of branches shown, two of which are 
quite small. The branches of each gland are arranged in groups, 
lettered in the figure A, B, C, and D. 
Folds. As stated before, irregular longitudinal folds of the 
mucous membrane are found in the ileum in stages ranging 
from 24mm. to 55mm. These folds, however, are only transient, 
and are in no way related to the folds found in the later stages and 
in the adult. They have practically disappeared at 55 mm. 
Circular folds, which have been studied from longitudinal sec- 
tions, are found in the small intestines of the older embryos. 
Whether these folds are formed by the contraction of the muscu- 
lature, or whether they are the non-effacable plicae circulares 
(valvulae conniventes) could not be positively determined from 
cut sections, but from their regularity and constaacy, I have con- 
sidered that they are the latter structures. Their presence was 
first noted in the mid region of the small intestine at 73 mm., but 
THE AMERICAN JOURNAL OF ANATOMY, vor. 10, No. 4, 
