Mucous Membrane in the Human Embryo. 541 
lumen of the duodenum is totally occluded. Below this region 
the vacuoles are fewer and the lumen from this point on is open. 
A few vacuoles are present in the upper part of the jejunum 
at this stage (16 mm.) but these are not as large or as distinct 
as those found in the duodenum. The lumen of the jejunum and 
ileum, unlike that of the duodenum, is pervious throughout its 
entire extent, although in the ileum it is much smaller. There 
have developed in the epithelial wall at this stage a number of 
outgrowths, which have been described as ‘‘intestinal divertic- 
ula.”’ These rounded, bud-like. structures are constricted at 
their necks, and extend into the surrounding mesenchyma. They 
are found in different stages of development and are variable in 
size. The smallest have diameters of .043 mm., the largest of 
about .072 mm., which is about 4 that of the small intestine itself. 
The older pockets are well marked and are six in number, the 
younger ones are not as distinct and a few are doubtful. These 
were 11 in number, making a total of 17 pockets in the small 
intestine of this embryo. 
Keibel (05) noted the presence of similar pockets in embryos 
of man, Tarsius, pig, and deer, and considered it strange that 
these structures were not mentioned by Voigt (99) and Berry 
(02). Lewis and Thyng (08) described diverticula which they 
found along the small intestine in human, pig, and rabbit embryos. 
In a human embryo of 23 mm. they counted 33 pockets; at 22 
mm. (H. E. C. series 851), 48 pockets. Elze (’09) found in various 
mammalian embryos, rounded and elongated intestinal pockets 
similar to those described by Lewis and Thyng. He observed 
that the pockets are always on the wall of the intestine away from 
the mesentery, and that those pockets which were elongated and 
extended along the intestine fora short distance, invariably pointed 
down the intestine (ab-orally). In the human embryos of the 
Harvard Collection, so far as they have been examined, the diver- 
ticula present the same characteristics. Because of differences 
in their arrangement and position, Elze puts the diverticula of the 
duodenum in a separate class from those of the jejunum and ileum. 
At 19 mm. (series 819) the vacuoles in the duodenum are larger 
than those at 16 mm. The lumen is continuous throughout and 
