RESPIRATORY ORaAXS IX ARAXE.E. 
19 
and 14) much raora than was the case ia the previous stage, 
where the groove was straight and transverse. The angle of 
the slanting surface varies, the latter being in some embryos 
nearly perpendicular, in others nearly parallel to the adjacent 
body surface and in the latter case the curved furrow 
may entirely disappear. The above will become clear by com- 
paring figs. 8b and 9 of this stage with the corresponding 
section (fig. 7) of the previous stage. 
In the second place we notice a little pocket-like cavity 
(palm. .s\, figs. 8 and 14) extending from the middle of the 
base behind in a latei’al direction for about one third of the 
breadth of the appendage. This cavity, which we may term 
the pulmonary sac, is practically all that remains of the 
once extensive post-appendicular groove, and is to be con- 
sidered as a portion of the latter which had become especially 
deepened and so escaped the obliteration which befel the rest 
of the groove — for a subsidence has also taken place in the 
tissue immediately behind the pocket. (Compare fig. 8g 
with the corresponding section, tig. 7a, of the previous 
stage.) 
The ])ulmonary sac was fii*st described and figured by 
.Metschnikoff (’71) for the scorpion and was found by most 
subsequent investigators, but generally in a later stage of 
development. 
'rhe cells which form the wall of the sac undergo from 
now on repeated division (fig. 8o), causing the sac to grow 
rapidly, at first in a forward direction in the form of an in- 
pushing under the appendage, but later on in a latero-dorsal 
or dorsal direction (fig. 10). The anterior wall of the pul- 
monary sac yields the cell-material for all the lung saccules, 
except the first two, whose appearance forms the third and 
most important point to be noticed in this stage. 
On the medial half of the posterior side of the appendage 
there appear two parallel furrows of varying length (/. 1,/. 2, 
figs. 8 and 14). These are the first beginnings of the two 
oldest saccules of the lung-book. They are never transverse 
but always incline to the longitudinal axis of the appendage 
