550 Franklin P. Johnson. 
stages they present a variety of different shapes in different parts 
of the intestine. The villi of the duodenum at 78 mm. are cylin- 
drical, with rounded ends, averaging .23 mm. in height. In the 
upper part of the jejunum they are blunter and measure .20 mm; 
in the mid region elongated and slender, .25 mm. in height; while 
in the lower part of the ileum they are swollen at their apices and 
measure about .28 mm. In the mid region at 134 mm. (fig. 23), 
the villi are pointed processes; at 187 mm. and 240 mm., they are 
‘more elongated. In the mid region at seven months (premature 
birth) irregular villi are found, some of which are foliate in shape 
and grooved on their sides. At birth, the villi of the mid region 
are likewise very irregular in shape. 
In briefly summarizing the development of villi, it may be 
said that the general tendency throughout the whole of the small 
intestine is for villi to develop as separate invaginations of the 
epithelium. Owing, however, to the occurrence of transitory 
structures (vacuoles, diverticula, and folds) their development is 
manifested differently in different parts of the intestine. In the 
duodenum the early growth of the villi is closely associated with 
and concealed by the vacuolated condition of the epithelium. 
While the vacuoles are present, the apices of the villi of one wall 
seem fused with those of the opposite wall. They become distinct 
when this fusion breaks down. At the same time the vacuoles 
are lost by entering into the formation of the lumen. Through- 
out the upper two thirds of the jejuno-ileum, villi are first seen as 
separate thickenings of the epithelium. These thickenings become 
invaginated, and the thickened plate of epithelium becomes 
reduced to a single layer of cells. In the lower third of the small 
intestine, longitudinal folds precede the formation of villi. How- 
ever, since the folds are present only in those regions of the intes- 
tine which are not expanded, they are probably formed because 
the outer coats of the intestine have not grown as rapidly as the 
epithelium. The irregularities seen along the tops of the folds, 
which are of about the same size as the slight swellings in the ex- 
panded portions of the gut, are probably developing villi. The 
folds may later disappear by a general expansion of the epithelial 
tube, or by being absorbed by the rapidly growing villi. I find 
