546 Franklin P. Johnson. 
enchyma.) Although the villi are somewhat more numerous in 
the duodenum, where the diameter of the epithelial tube is large, 
they are also foundinthe greater part of the jejunum. Following 
down the jejunum, the larger villi become fewer and fewer until 
a region is reached which shows only thickenings. Below this 
point the lumen of the intestine is smaller and rounded and re- 
mains this way until the large intestine is reached. 
At 22.8 mm. the villi are larger and more numerous than at 
19 mm. They are, as shown in fig. 15, irregularly shaped 
processes. Below the openings of the ducts, villi—if it can be 
assumed that they are present—are not recognizable from an 
inner view Of the gut cavity. This is due to the occlusion of 
the duodenal lumen and to the occurrence of the large vacuoles. 
However, the outer surface of the epithelium of this region is 
marked with pits filled with mesenchyma. These pits are simi- 
lar to those forming the cores of distinct villi found in the other 
parts of the intestine. It can be inferred from this, therefore, 
that the formation of villi is taking place while the vacuoles 
are present’in the epithelium, but the individual villi are not 
distinguishable during the solid stage of the duodenum. 
Just below the vacuolated region of the duodenum, true villi 
are found at this stage (22.8 mm.). They extend into the first 
loop of the jejunum, but the villi of the duodenum, having devel- 
oped first, are the larger. As the jejunum is followed down, the 
villi become smaller and smaller, until a level is reached in which 
villi are absent. The villi developing in this lower portion of the 
small intestine are as young as any in the whole gut. In order to 
makeanaccurate study of their development, a wax reconstruction 
was carefully made of thisregion. Fig. 18 shows half of the model, 
a, looking at the outer surface of the epithelium; 6, at its inner 
surface. An examination of the inner surface shows the presence 
of a number of rounded, knob-like structures, the beginnings of 
the villi. These are only seen in the upper part of the model. 
Thickenings of the epithelium are seen in a region below the villi, 
but at the extreme end of the tube there are no evidences of villi 
at all. By comparing the external surface of the epithelium, a, 
with the internal surface, b, it isseen that there are depressions on 
