RESPIRATORY SPACES OF THE LUNGS 357 
originates from one kind of cells and develops into two kinds 
with the beginning of respiration, but Minot and Miller have 
quite an opposite idea. Miiller studied the still-born embryos 
of horses as well as oxen and a 230-day ox embryo. He did not 
find an unbroken coating of epithelium on the walls of the 
alveoli, but observed gaps between the nucleated cells, which 
seemed to signify the presence of the non-nucleated cells. He 
believed it impossible to explain the formation of non-nucleated 
cells by the act of respiration only, but supposed that the cells 
are formed in the later embryonic stages. Miller only used 
ordinary stained preparations which are unsuitable for the study 
of respiratory epithelium, and he did not use the silver method, 
so that his results cannot be regarded as conclusive. From this 
diversity of opinion it is evident that the question of embryonic 
respiratory epithelium and its formation is still under controversy 
and that an answer has not yet been given as to when and how 
the large, flat cells lose their nucle. 
For my investigations I used rabbits for the most part, also 
guinea-pigs, rats, and mice. 
No impregnation occurs in embryonic tissue if freshly cut 
pieces are soaked in the solution of silver nitrate under pressure, 
so that for impregnation the solution was injected through the 
trachea, resulting in the expansion of alveoli. In general, impreg- 
nation of the embryonic respiratory epithelium cannot be easily 
accomplished. 
I impregnated 11-em. rabbit embryos and noticed that the 
respiratory epithelium was differentiated into two kinds, small 
cells and large, flat cells, but this difference seems to be the 
result of the expansion of the alveolar walls by the silver injection. 
The small cells have nuclei and significantly the large, flat cells 
also contain nuclei. The nucleus takes up a greater part of the 
cell body in the small cell, while it is located either in the middle 
or eccentrically (fig. 13). The lungs of the animals of the same 
litter were used as controls, some being merely soaked in the 
fixing solution, while others were injected with the fixing solution 
into the trachea. In such preparations it can be seen in surface 
view that some of the alveoli are lined by a uniform cuboidal epi- 
