Mucous Membrane in the Human Embryo. 547 
the outer surface to correspond with the villi. The thickenings 
of the epithelium may or may not have corresponding slight 
depressions. This model indicates that each villus originates as 
a thickening of the epithelium, and that later this thickening is 
invaginated.- Still later the epithelium of the invagination becomes 
thinner and is reduced to a single layer of cells. Below the 
region of thickenings, the lumen of the intestine becomes smaller 
and rounded. Inno place could longitudinal folds of the epithe- 
lium be found. 
The idea that villi formation goes on simultaneously in that 
portion of the small intestine (duodenum) which is vacuolated 
and in that portion which is not, is further supported by the condi- 
tion found in the duodenum at 24 mm. In this embryo, only the 
lower portion of the duodenum shows vacuoles. In theupper part 
of that region which was previously occupied by vacuoles, villi 
are easily recognized. Further down there is a region in which 
are seen a number of villi which are fused together at their apices, 
that is, end to end. In some places this fusion has the form of 
thin membranes extending from villus to villus. The formation 
of the villi seems to be taking place by a pulling apart of preformed 
villi, and in this process the vacuoles are lost by becoming a part 
of the lumen. The process of separation of the villi progresses 
from above downward. 
In the mid region of the small intestine at 24 mm. (fig. 19), 
the villi are seen as separate knob-like elevations of various shapes 
and sizes. For the most part they have nearly circular ‘bases, 
but some are elongated. Their apices are rounded. The average 
of a number of measurement shows the breadth of a villus to 
be about .11 mm. The tallest villi found in this region measured 
12mm. The epithelium on the tops of the villi is single layered, 
although in places it appears pseudo-stratified, and is .020 to 
.025mm.in thickness. Between the villi are epithelial thickenings 
which measure .064 mm. in thickness and are composed of from 
3 to 5 layers of cells. An examination of the whole of the model 
(half of which is seen in fig. 19) shows that the villi are arranged 
in five longitudinal rows. In the lower part of the intestine the 
villi are smaller; still lower down only epithelial thickenings occur. 
