RESPIRATORY SPACES OF THE LUNGS 361 
I will now describe the cause of the formation of the two kinds 
of respiratory epithelial cells. According to Kiuttner, the for- 
mation of two kinds of cells is due to the pressure of air alone at 
the time of the inflation of the lung. However, as the alveolar 
walls are exposed to the same pressure at all points, it would 
seem that the epithelial cells would all become equally flattened 
unless there were some other cause at work, so that his opinion 
seems open to objection. Jalan de la Croix insists that instead 
of pressure the tension must be considered as the cause, but it 
seems to me that this is open to the same objection, as pressure 
and tension are merely different expressions of the same condi- 
tion. Besides the tension, the existence of blood-capillaries is 
looked upon as another cause by Schulze, Croix, and Kolliker. 
According to Kélliker, the epithelial cells which coat the blood- 
capillaries will be particularly subject to strain and flattening 
by the expansion of the alveolar walls. All these investigators 
made these assumptions on the ground that the respiratory 
epithelial cells are homogeneous in the embryonic stage, but, as 
I explained above, the epithelial cells are already differentiated 
before respiration into two kinds, so that these conclusions are 
quite unsatisfactory. It seems to me that the best explanation 
is as follows: In the alveolar walls of the embryos no basal 
membranes are visible below the epithelium and the epithelial 
cells are located directly on the blood-capillaries. Therefore, 
they are gradually extended and flattened by the growing of the 
alveolar walls on one hand and of the blood-capillaries on the 
other hand, especially on the blood-capillaries. It can be easily 
imagined that as the epithelial cells are flattened, they acquire a 
great power of extension compared with the inter-capillary cells, 
so that, when respiration takes place and the alveoli are suddenly 
expanded, the flat cells become conspicuously extended. Whether 
or not this statement is accepted as correct, there can be no 
doubt about the fact that by some process a part of the epithelial 
cells become differentiated during later embryonic stages in such 
a way that they are more capable of expansion. Moreover, the 
extensibility of epithelial cells seems to have nothing to do with 
the disappearance of their nuclei, for the epithelial cells can be 
