24 
W. F. PURCELL. 
hounded hy the original cylindrical cells (figs. 8b and 8c). 
In these two figures we see, however, the commencement of a 
second internal cleft {cl. 2) and a second external furrow, 
the latter being the second pulmonary fur row (/. 2). In a 
later stage the cells betAveen the second internal cleft and 
the two external furrows are seen in the process of shorten- 
ing' to one half of their former length in order to re-arrange 
themselves to epithelia having this cleft and the two furrows 
for their basal and free surfaces respectively (figs. 10 and 
11 ). 
d'he walls of the oldest saccule, embracing the first pul- 
monary furrow between them, and the distant Avail of the 
second saccule, are, therefore, uoav present. In a similar 
manner the proximal wall of the second saccule and the Avails 
of all subsequent saccules are formed (fig. 15). 
The external pulmonary furroAvs are ahvays provided Avith 
a distinct lumen, cutting deep into the sides of the appendage 
and sac, and they have been thus figured by Simmons, who 
Avas, I believe, the first to observe them. The internal clefts, 
on the contrary, have no real lumen AvliateA^er, and are 
indicated on the visceral surface of the epithelium by slight 
grooves only. 
The process described above may be summarised as follows : 
The cells of the epithelial region Avhich contains the 
pulmonary furroAvs shorten and re-arrange them- 
selves in the form of a folded epithelium, Avhich has 
one half the thickness of the original epithelium 
and occupies about the same total A’^oliime. 
Ily re])eated cell-division the folded e[)ithelium expands 
in such a manner that all those folds Avhich are directed 
iiiAvards and Avhich contain a pulmonary furroAv between 
their Avails groAv into and ultimately fill out the cavity of the 
appendage (figs. 10 and 16 a). Each holloAv pouch thus 
pn'oduced gives rise, of course, ultimately to a hollow air- 
containing saccule, Avhile the outAvardly directed folds com- 
prised betAveen tAVO pulmonary furroAvs ultimately form the 
septa betAveen the air-compartments of the lung-book. The 
