OF 
Mucous Membrane in the Human Embryo. Sao 
stratified, is now becoming one-layered. This change is brought 
about by a readjustment of the cells, caused—first, by the devel- 
opment of the pits; second, by the development of somewhat 
similar pits or furrows on the outer surface of the epithelium. 
These latter structures are found alternating with the pits of the 
inner surface and are in different stages of development. Some 
appear as mere slits in the epithelium, others are already broader 
than many of the gastric pits themselves. Whether these pits 
are formed by an ingrowth of the mesenchyma, or whether they 
are formed independently of mesenchyma by a readjustment of 
the epithelial cells, is difficult to determine. In the specimens 
studied, the mesenchyma had shrunken away from the epithe- 
lium, but from their extreme depth and narrowness it is-hardly 
probable that the mesenchyma could have extended into the nar- 
rowest of these furrows. However formed, these furrows cause 
the epithelium to be transformed into a simple epithelium. The 
cells of the surface epithelium and those about the pits give off 
slender basal processes. These cells have been described by 
Baginsky (82) at 7 months, and by Fischl (91) at birth. The 
cells have distinct cell boundaries, rounded nuclei, are granular, 
and show distinct top plates. 
At 91 mm. the grooves on the mesenchymal side of the epithe- 
lium are well developed. In a model of the epithelium at this 
stage (not figured) these mesenchymal grooves have about the 
same appearance as the gastric pits, so that one can hardly deter- 
mine which is the internal and which the external surface. The 
epithelium is composed of a single layer of cells. The cells are now 
higher, with more elongated nuclei, and have long basal processes. 
Some of the cells are clearer than others and seem to be filled 
with mucous secretion. 
At 120 mm. the gastric pits have increased in number and in 
size, and are slightly more separated from one another than before. 
The growth of the epithelium has taken place both by an enlarge- 
ment of the pits already formed and by the addition of other 
pits. A wax reconstruction of the epithelium of the fundus of 
the stomach at this stage (fig. 11) compared with fig. 10 (55 mm.) 
will give a clearer idea of the growth of the pits. The arrangement 
