356 CHIKANOSUKE OGAWA 
3. Aecording to my observation in regard to the formation 
of the respiratory epithelium, non-nucleated cells arise from 
nucleated cells by the loss of their nuclei, the details of which 
process will follow in the next section (D). 
In short, from the above argument, it seems that the non- 
nucleated cells cannot be considered as parts of the nucleated 
cells. 
D. Respiratory epithelium in embryonic and new-born mam- 
mals. No agreement has hitherto been reached in the discus- 
sion as to the form of this tissue in foetal life and its change 
at birth. Kiittner makes seemingly contrary statements in his 
work, on the one hand describing the respiratory epithelium of 
the embryo as gradually flattening out up to the time of birth, 
and on the other hand speaking of the cuboidal cells of the 
embryonic alveolus as changing to the flat cells with the first 
respiration. Stéhr seems to be of this same opinion concerning 
the sudden change of the flat cell at the first breath. Stieda 
showed that in the lung of a 250-mm. sheep embryo and of a 
large ox embryo the alveolar ducts are coated with a simple flat 
epithelium. Kdlliker also finds a homogeneous lining of flat 
epithelium in the alveoli of mature embryos, and according to 
him the respiratory epithelium is formed in such a way that the 
epithelial cells which cover the blood-capillaries are subject to 
great pressure by the mechanical expansion of the alveolus at 
the first breath and consequently are stretched considerably. 
Schulze describes the alveolar epithelium of fully developed 
embryos as flat, and he claims that the formation of non-nucleated 
cells is due to the expansion of the blood-capillaries of the alveolus 
and that the fusion of adjacent cells also participates in this 
formation. Ebner believes that the large, flat cells arise from 
the fusion of embryonic flat cells. According to Croix, the 
alveolar epithelium of the human embryo before respiration has 
taken place is all flat and two kinds of epithelium are formed in 
the same way that Schulze describes, only without the process 
of cell-fusion. Moreover, Croix asserts the loss of the nucleus 
from the large cells is ascribed to their flattening. All above- 
mentioned authors consider that the respiratory epithelium 
