120 Proceedings of Royal Society of Edinburgh. [sess. 
Those portions of the groove where less friction occurs are lined 
by a form of epithelium transitional between the ciliated and the 
stratified squamous varieties. 
This led us to believe that friction was the determining agent in 
forming the squamous epithelium found in these situations. 
In order to determine whether or not the squamous epithelium is 
produced from ciliated epithelium present in those situations where 
friction eventually occurs, we made sections of the trachea of a 
foetal kitten, 5 cm. long. 
The cartilage rings were found to surround the trachea for only 
three-fourths of its circumference, the trachealis muscle being well 
developed posteriorly, and the mucous membrane being thrown into 
folds beneath it. The mucous membrane was lined by a stratified 
epithelium, the surface cells of which were columnar, and devoid of 
cilia. 
We then cut sections from a kitten two weeks old, and found 
that the cartilage now completely encircled the trachea, the ends of 
the plates, however, not overriding as yet, though there were indica- 
tions of one end projecting inwards into a fold of the mucous 
membrane. The epithelium lining the mucous membrane was seen 
to be stratified and ciliated in all positions. 
It appears, therefore, that the stratified squamous and transi- 
tional forms of epithelia present in the adult trachea result from 
a modification of ciliated epithelium of the ordinary type, due to 
the influence of friction exerted during the animal’s life. 
Still more remarkable changes have been described by biologists. 
Of Hunters’ Gull there is no histological record. But in other cases 
there is abundant testimony to the mutability of one form of cell 
into another ; as, for example, the transformation of osteoblasts into 
osteoclasts, and vice versa. 
In such cases the cause is unknown, but is probably environ- 
mental. 
But in the case which we have described there can be no doubt 
that the change is brought about by the agency of friction occurring 
as a physiological phenomenon during the life of the animal. This 
induces the cells in the deeper layers of the epithelium to take on a 
new line of development, producing flat keratinised cells in place of 
ciliated ones. 
