RESPIRATORY SPACES OF THE LUNGS 339 
than ten times the diameter of the blood-capillaries. The inter- 
capillary spaces are not grouped in the lung according to their 
shape and size, but, on the contrary, they may present various 
appearances even in the same alveolus. 
When the intercapillary spaces are narrow, they are, as already 
mentioned by Elenz, filled up entirely by small cells, while the 
slit-like spaces are lined only partly by small cells and for the 
most part by flat ones; occasionally the small cells do not lie 
entirely in the intercapillary spaces, but are somewhat removed 
from them. Moreover, the intercapillary spaces are sometimes 
not filled by small cells, but by large, flat cells alone. 
In surface view the large, flat cells show a square to hexagonal 
form and their diameters are from four to five times as large as 
those of the small cells. Altogether in Amphibia both the cement 
lines and the nuclei of the respiratory epithelium turn black by 
impregnation, in the tortoise only the cement lines are impreg- 
nated. As previously mentioned, Elenz was not able to demon- 
strate nuclei in the flat cells. In order to clear this question, I 
stained silver preparations with nuclear dyes and studied them 
in surface view, but since the nuclei of subepithelial cells were 
stained at the same time, it was difficult to judge, even with 
the greatest care, whether or not the flat cells contained nuclei. 
Subsequently numerous celloidin sections were made from such 
preparations and the point at which the epithelial cells were cut 
off tangentially from the underlying tissue was carefully ex- 
amined. Here, where the epithelial cells were free, their nuclei 
could be seen lying within blackened cement lines, either round 
or elliptic, and located sometimes in the centre, sometimes 
eccentrically (fig. 5). These observations show that Elenz’s 
negative results were due to inadequate technique. As a large, 
flat cell often covers a number of intereapillary spaces at the 
same time, spaces containing no nucleus are not as rare as in 
Diemyctylus. 
Elaphe quadrivirgata (snake). As far as I know, Elenz is the 
only one who has given a detailed statement of the respiratory 
epithelium of the snake, which may be quoted as follows: 
