RESPIRATORY SPACES OF THE LUNGS 359 
the birth. The silver preparations of these new-born animals 
appear almost the same as those of adult animals and no nuclei 
are visible in the flat cells (fig. 16). The flat cells were examined 
in the way mentioned above, and though I could find places in 
which the absence of nuclei was perfectly certain, it was tech- 
nically impossible to show that all of the flat cells in the field 
were non-nucleated. The stained preparations also presented 
the same appearance as those of the full-term embryos (fig. 17). 
It is clear from these descriptions that the respiratory epithe- 
lium of the embryo in early stages consists of a single kind of 
cuboidal cell and, as development proceeds and comes nearer to 
the final stage, some of them become flatter. This differentia- 
tion does not occur to the same extent in all alveoli, some alveoli 
being covered by cuboidal and flat epithelial cells and others by 
cuboidal cells alone. In the final stage the respiratory epithe- 
lium of all the alveoli consists of a mixture of the two kinds of 
cells. Therefore, the prevailing opinion that respiratory epithe- 
lial cells are not differentiated into two kinds until the first respi- 
ration is not true. But it is beyond doubt that the flat cells, 
which have differentiated before the beginning of respiration, 
become flatter in consequence of respiration. 
Disappearance of the nuclei of flat cells takes place also in the 
final embryonic stages, and occurs not suddenly, but gradually. 
It can be inferred from preparations in silver and ordinary stains 
that the greater part of the nuclei probably has disappeared before 
parturition. 
Now I will describe the process of disappearance of nuclei. 
As can be seen in figure 14, the nuclei of some of the flat cells are 
stained more deeply than those of other adjacent cells and are 
smaller in size and irregular in shape, not being round. Some- 
times chromatin substances are assembled at one side of the nuclei 
and the free edges of the nuclei are irregular. Moreover, they 
are often stained by eosin. These different nuclear conditions, 
which cannot be considered as normal, correspond to the phe- 
nomena of pyknosis and karyorrhexis and may be interpreted 
as being in the process of disappearance. In the new-born 
foetus we sometimes find the same suggestion of nuclear disap- 
pearance in alveolar walls. 
