RESPIRATORY SPACES OF THE LUNGS 345 
perceived, as the whole cell body was deeply impregnated. On 
the other hand, nuclei cannot be seen in either kind of cell in 
the lung of the Gecko in this way, but by using the method 
described above in connection with Elaphe, I was able to prove 
the existence of a nucleus in both kinds of cells. The nuclei 
of the two kinds of cells are of the same magnitude in surface 
view and round or elliptical in shape, the nuclei of the large 
cells are usually located near the capillaries, in the middle or 
near the border of the cells themselves. 
To sum up the relation of the epithelium to the intercapillary 
space, it may be said that an intercapillary space is covered either 
by the small cells or the large ones or by both kinds of cells. 
3. Comparison of the respiratory epithelium in Amphibia and 
Reptilia 
I have become convinced that the respiratory epithelium of 
Amphibia consists of a single kind of cell, while that of Reptilia 
is made up of two kinds. Some authorities, such as Schulze 
and Osawa, however, regard the epithelium of the two forms 
as similar, so that perhaps a thorough examination of the differ- 
ences between the two will not be out of place. 
The large flat cells of Reptilia correspond to the cells which 
have a flat as well as a nucleated portion in Amphibia, while 
the smail cells of Reptilia might possibly be compared to the 
small cells of Amphibia which have no flat portions. These 
cells, however, are so few in number that they can hardly be 
considered as a regular form, and it is safer to say that there are 
no cells which exactly correspond to the small cells of Reptilia. 
The large flat cells of Reptilia appear at first sight like the 
epithelial cells of Amphibia. On closer inspection they are found 
to have differences. In a cross-section of an alveolus of the 
amphibian lung, the nucleated portion of the respiratory epi- 
thelium, lying in the intercapillary space, has the same height 
as that of the capillary, and the shape, though varying in different 
parts and at different stages of expansion, may be described as 
cuboidal, while the nucleus presents a spherical form. In 
Reptilia the case is different. In the tortoise, for example, the 
