Mucous Membrane in the Human Embryo. 523 
being no sharp line of demarcation between the two. The indis- 
tinctness of the cell boundaries makesit very difficult to determine 
definitely the number of cell Jayers in the epithelium. Because of 
this, different observers have not agreed upon the number of cell 
layers present in the early stages. It can, however, be said with 
reasonable certainty that the epithelium at 7.5 mm. is strati- 
fied and in the upper and lower thirds of the cesophagus shows 
three or four rows of nuclei, in the middle third, two or three rows. 
The basal cells are the more columnar and are characterized by 
having their nuclei in that end of the cell which is toward the 
lumen. Surrounding this central tube of epithelium is mesen- 
chyma, which although undifferentiated is slightly condensed. 
In an embryo of 16 mm. the oesophagus has increased in length 
and breadth. The upper end is both larger and more irregular 
in outline. The epithelium contains three or four rows of nuclei 
and lies on a very distinct basement membrane. Here again, the 
basal layer of celJs is the more columnar and its rounded nuclei 
lie in the upper ends of the cells, that is, away from the basement 
membrane. In its lower portion, the cesophagus is smaller and 
ovalin cross section. Throughout its entire extent the cesophageal 
epithelium is surrounded by mesenchyma which is limited exter- 
nally by a narrow ring of deeply staining myoblasts, elongated 
in form, and so placed as to form the circular layer of the muscu- 
aris. 
Vacuoles. The next stage examined was an embryo of 19 mm. 
(series 819). In this only about the lower half of the cesophagus 
was available for study. The epithelial tube is now irregular 
in outline, and presents within its walls numerous small cavities 
or vacuoles. Some of these are minute, others have a diameter 
0.051 mm., which is greater than the diameter of the lumen itself, 
making it difficult to determine from cross sections which is lumen 
and which vacuole. In some places the vacuoles open directly 
into the lumen, in other places they are separated from it by a 
partition of epithelium. Fig. la is of a model of the cesophageal 
epithelium at this stage and shows its vacuolated structure. 
The portion of the oesophagus modelled was selected at a place 
just below the bifurcation of the trachea. In a second embryo 
