Mucous Membrane in the Human Embryo. 551 
no evidence that there is a mechanical cutting up of the folds into 
villi, such as has been described. 
The villi of all parts of the intestine later become arranged in 
more or less regular longitudinal rows. Additional villi develop 
as separate growths by forming new rows, and also in between 
the villi of the older rows. In some places their development 
has gone on so rapidly that the villi of a single row are not entirely 
~ separate, and give the appearance of short, irregular longitudinal 
folds. This appearance is lost in the later stages. 
In any single embryo, the larger villi, which are all approxi- 
mately the same size, are always found to be more numerous than 
the smaller ones. This indicates that the growth of the younger 
villi is more rapid than that of the older ones. After they have 
attained the size of the older ones, however, they develop at a 
corresponding rate. 
The growth of the villi in the duodenum and mid region of 
the small intestine is shown in Tables 8 and 4. The villi of the duo- 
denum, having developed first, are the taller in the earlier stages. 
At 55 mm. the villi in the mid region have attained the height 
of those in the duodenum and from this stage on are larger. The 
bases of the villi remain approximately constant in size but show , 
a slight initial increase and later a diminution in their diameters. 
The epithelium on the tops of the villi, which is at first stratified, 
then simple columnar, and finally simple cuboidal, is much re- 
duced in thickness. 
Intestinal Glands. According to Koelliker (61 and ’79) the 
intestinal glands (Lieberkiihn’s crypts) arise as hollow out- 
growths of the epithelium which push their way into the meso- 
derm. Barth (’68) did not accept this view of the origin of the 
glands, but believed that they are formed by the spaces left in 
between the villi. Brand (77) believed that an upward growth 
of the mesenchyma was the active factor in the development of 
the glands. Minot (92) and Kollman (’98) agree with Koelliker. 
Voigt (99) studied the development of the intestinal glands in 
the pig embryo, and likewise concluded that they originate as 
hollow outgrowths of the epithelium which are given off between 
