Mucous Membrane in the Human Embryo. 548 
uring .084 mm. Three of these pockets are seen in fig. 17. The 
first one (lettered a) is constricted from the intestine on all sides. 
The other two (6 and c) are elongated forms, and are not entirely 
constricted at their necks, b being a surface view, and ¢ an end 
view. The two latter extend along the intestine and their free 
ends point ab-orally. 
At 24 mm. the vacuoles have disappeared from the upper 
half of the duodenum, but they are still present in the lower half. 
In the upper half, well formed villi are present; in the lower half, 
villi are seen which are fused together at their apices, but spaces 
are seen between adjacent villi. These spaces are the above de- 
scribed vacuoles. It is evident from this that the disappearance 
of the vacuoles is associated with the formation of villi. The 
exact part played by the vacuoles in the formation of villi will be 
dealt with in connection with the latter structures. In the duo- 
denum of an embryo of 30 mm. the vacuoles have entirely dis- 
appeared. 
Because of the development of villi, the intestinal diverticula 
are difficult to count at 24.mm. It is due to these beginning 
villi that the pockets disappear. Villi arise around the mouth 
of a pocket, and by their subsequent growth and enlargement, 
the pocket is gradually lost by being absorbed in their walls. 
Some pockets are more elongated than others, and on either side 
of these short longitudinal folds are seen. By a growth of these 
folds, pockets are likewise obliterated. As the villi develop from 
above downward, the pockets disappear in the same order. Con- 
sequently, in embryos of 29 mm., 30 mm., 32 mm., and 42 mm., 
the number of intestinal diverticula becomes smaller and smaller. 
Those in the ileum attain the highest development. As they in- 
crease in size their outer surfaces become flattened and later may 
even become concave. At 30 mm., the diameter of one of the 
largest pockets measures .108 mm ; at 37 mm. .176 mm. 
- The significance and fate of the diverticula have been discussed 
by Lewis and Thyng. They conclude that some elongated forms, 
which are usually found in the duodenum, are the source of an 
occasional accessory pancreas. The more rounded intestinal 
pockets usually degenerate, but some may remain to form cysts 
