526 Franklin P. Johnson. 
before others and show different stages of development in different 
regions of the same oesophagus. The exact time and order of the 
appearance of these structures was studied in a number of embryos 
with the following results. 
As stated before, the epithelial tube of the cesophagus at 7.5 
mm. is cylindrical in shape, very small at its mid-region, but 
gradually becoming larger when followed either up or down. 
In embryos of 10 mm. and 16 mm. the upper portion of the 
cesophagus is flattened ventro-dorsally, the middleregion is circular 
but now more expanded, and the lower portion is flattened later- 
ally. In the upper part of the cesophagus there is a slight infold- 
of the ventral wall of the epithelial tube. 
In the upper part of the cesophagus at 22.8 mm. (series 871) 
there is a ventral infolding of the epithelium. This is the direct 
downward continuation of that fold of the pharynx which gives 
to the latter, when seen in cross section, its characteristic cres- 
centic shape. Following downward this fold is soon lost and the 
cesophagus becomes rounded. Another fold appears in the mid- 
region of cesophagus, this being an infolding from the dorsal side. 
Still further caudally, as the stomach is neared, a third infolding 
is found, this being left lateral in position. This lateral fold in 
an older embryo is seen to be directly continuous with the dorsal 
fold above. The folds found in this embryo are, therefore, really 
two in number, a ventral one which appears in the upper third. 
of the cesophagus, and a dorsal one which is seen in the middle 
third and again as a lateral fold in the lower third. Since the. 
study of the order of the appearance of these folds was, made. 
by wax reconstructions in a series of embryos, it was necessary. to 
secure for modelling approximately the same level of the ceesopha- 
gus in each specimen. The region selected was that immediately 
below the bifurcation of the trachea. ‘4 
The cesophagus at 37 mm. in the region of the bifurcation of 
the trachea (fig. 2) is somewhat crescentic in cross section. Dor-, 
sally, facing the dorsal aorta, there is a deep fold which continues, 
upward and downward and passes directly onto the stomach, 
In the lower third, however, it deepens and changes its position. 
by moving through an arc of about 90° to the right. At 42 mm, 
