Mucous Membrane in the Human Embryo. 527 
in the same region as the above (fig. 3) there are two distinct 
folds, a dorsal and a ventral. These folds, which I have designated 
as the first pair of primary folds, are continued downwards onto 
the stomach, but before reaching it are twisted to the right and 
occupy positions on the left and right sides of the cesophagus 
respectively. In the figure a second pair of primary folds can be 
seen, which, although indistinct at this level, are well marked below. 
In the region modelled and below it, the fold on the left side is 
the more marked, but above the right fold alone is seen, and the 
ventral and dorsal folds approach one another on the left side, 
making a three sided figure. At the extreme upper end of the 
oesophagus these folds become irregular and are broken up by the 
formation of other smaller folds. All the folds shown in fieeD 
are twisted as the stomach is neared, each occupying a position 
of about 90° to the right of its original position. 
At 55 mm., both pairs of primary folds are well developed 
(fig. 4) and give to the lumen of the cesophagus in cross section 
the appearance of a Greek cross. These folds are also seen in em- 
bryos of 30 mm. and 42 mm. in the lower parts of the oesophagus. 
As seen in fig. 5, at 134 mm., these primary folds are augmented 
by secondary folds which develop at the bases of the former. 
Three such folds are seen in fig. 4, one of which is quite large. 
The same folds are found in embryos of 187 mm., 240 mm., and 
at birth. 
By way of summary concerning the development of folds of 
the mucous membrane of the cesophagus, it may be said that the 
dorsal one is the first to develop completely. This is followed 
closely by the ventral fold. The second pair of primary. folds 
develop from below upwards and have reached to the bifurcation 
of the trachea in,an embryo of 42 mm. The secondary folds 
are less constant structures. They begin as thickenings of the epi- 
thelium between the bases of adjacent primary folds. In the upper 
part of the cesophagus, especially in the older stages, folds are 
very irregular. The reader is referred to Table 1, page.532. for 
measurements of folds of the cesophagus in different stages of 
development. | 
The fact that the epithelial folds of the cesophagus rotate 
