308 CHIKANOSUKE OGAWA 
thelium and others are coated by epithelium which is differ- 
entiated into cells of two different shapes (fig. 14). Also in 
surface view the nuclei of the epithelial cells are seen separated 
from each other and between them the red blood-corpuscles are 
often apparent, implying the presence of blood-vessels in these 
spaces. When looking at the sectioned surface of the alveolar 
wall, it is often impossible to distinguish any epithelial cells on 
the blood-capillaries, which shows that very thin portions of 
epithelium exist here. 
I also removed the embryos from the mother animals just 
before birth and treated the lungs in the same way. I found 
here also that the epithelial cells were differentiated into two 
kinds. The flat cells are partly non-nucleated and partly nucle- 
ated. Whether or not a cell contains a nucleus requires very 
careful consideration, though it is less difficult here, because the 
alveolar walls of embryonic lungs are somewhat thicker than 
those of the adult animals. Cells in which no nuclei could be 
impregnated by silver and also distinguished by examining in 
a less refractive medium like glycerin under regulation of light 
were considered to have no nuclei. When a nucleus was not 
impregnated, but appeared within the border line of a cell by 
light adjustment, it was decided by careful focusing whether it 
belonged to an epithelial or subepithelial cell. But even then 
it was sometimes impossible to decide. I concluded that an 
epithelial cell contained a nucleus, when an impregnated nucleus 
appeared within the border lines of the cell. I then proceeded 
to fix and stain the embryonic lung of the same litter and found 
the microscopical image different from that of an 11-em. embryo. 
The blood-capillaries in the walls of the alveoli were filled by 
blood-corpuscles, though respiration had not occurred, and ac- 
cordingly could be traced without any injection. Seen in surface 
view (fig. 15) and at the cut plane, the nuclei rarely appeared 
on the blood-capillaries. In the intercapillary spaces there 
existed cuboidal, nucleated cells, which were nucleated epi- 
thelial cells. The border lines of the flat cells had disappeared 
in these stained preparations. I next tried silver impregnation 
and ordinary staining on the lungs of new-born rabbits just after 
