540 Franklin P. Johnson. 
The small intestine in the early stages of development of the 
human embryo is lined with an epithelium which is similar to 
that of the cesophagus and the stomach. This epithelium is 
composed of from two to four layers of cells surrounding the cen- 
tral lumen, which in the earlier stages is pervious throughout. 
The portion of the small intestine which is to become the duode- 
num begins to grow more rapidly than the lower portion, and in 
any single embryo may be seen to have attained a higher degree 
of development. In an embryo of 7.5 mm. the diameter of the 
epithelial tube of the duodenum is about twice that of the ileum, 
and its epithelial wall is much thicker. In the older stages, 
however, this advance in growth of the duodenum is not re- 
tained. 
In an embryo of 10 mm. the posterior wall of the duodenum is 
thicker than the anterior and contains for a short distance both 
above and below the entrance of the ductus choledochus, numerous 
small vacuoles. Beyond this opening the lumen of the duodenum 
becomes extremely small, and only a few vacuoles are present. 
The remainder of the intestine is smaller than the duodenum at 
this stage, and is more regular and round. Its epithelium, which 
is composed of three to four layers of cells and is about half the 
thickness of the duodenum, does not contain. vacuoles. 
Tandler (00) described similar vacuoles in the duodenum of 
embryos of from 30 to 60 days. He found that their formation 
in some cases led to a complete occlusion of the duodenal lumen, 
and believed that failure of the lumen to open again caused the 
well-known anomaly of duodenal atresia. Minot (’00) described 
a like condition in the large intestine of the chick. It is evident 
from his drawings (figs. 3-5) that the lumen of the large intestine 
of the chick is more completely occluded than that of the human 
duodenum. Forssner (’07) found similar vacuoles and duodenal 
occlusions in human embryos of about 20 mm. and also in the 
duodenum of rat embryos. 
At 16 mm. the epithelium throughout the entire extent of the 
duodenum is filled with vacuoles which are larger and more 
numerous than in the former stage. Immediately below the 
openings of the ducts, the vacuoles are most numerous and the 
