R. R. Bensley 143 
limits of the ordinal groups. Any similarity that may exist between 
various orders in the direction and nature of this differentiation, is 
probably simply a parallelism due to the operation of similar causes. 
Turning now to the question of the origin of the cesophageal divisions 
of the stomach, we find that there are two possible explanations put 
forward of which the first is that they are formed by the dilation of the 
lower end of the cesophagus. 
The arguments in favor of this view are in brief as follows: ; 
In nearly all cases where the cesophageal sac is of large dimensions it 
is traversed from the opening of the cesophagus to the glandular divi- 
sion by a continuous groove called the cesophageal groove. This has 
been interpreted as the direct continuation of the cesophageal tube, and 
it has been suggested that the esophageal sac has been produced by a 
hernia-like protrusion of that 
portion of the cesophagus. On 
this assumption Ellenberger, 
88, and Edelmann, 89, include 
these cesophageal sacs and the 
cesophagus together in the 
fore-gut (Vorderdarm), the 
true glandular stomach, ac- 
cording to these two authors, 
forming a part of the mid-gut 
(Mitteldarm). The second 
reason is based on the sup- Fra. 16. Stomach of Manis javanica (modified 
posed specificity of the epi- *™°™ YePe”- 
thelium, which is in all respects similar to that of the cesophagus. 
In view of the great plasticity of epithelium of a low grade of special- 
ization, little importance can be attached to the argument because, 
both pathological histology and normal histology afford many in- 
stances of the transformation of a cylindrical or a ciliated epithelium 
into a stratified epithelium, as a result of the operation of simple 
mechanical causes, such as friction. Such an instance has been noted 
by Haycraft and Carlier, 89-90, in the trachea of the cat. Here 
owing to the peculiar shape of the ring of cartilage, and the mode 
of insertion of the muscular layer, there is a permanent fold of the 
mucous membrane at the back of the trachea. As a result of the con- 
stant friction between the two layers of epithelium behind this fold 
the cilia are to a large extent lost and the number of layers of cells 
ereatly increased so as to bear a great resemblance to a stratified squa- 
mous epithelium. 
