1889. ] THE BODY-CAVITY IN LIZARDS, ETC. 457 
becomes observable as a bronchial cavity at the outer postero-dorsal 
corner of the lung. It lies in the mesoblastic tissue, which later 
develops into the avian diaphragm (‘‘ pulmonary aponeurosis ” + 
‘oblique septum”), but which at this date cannot be distinctly 
marked off from the mesoblast of the lung itself. 
During the first half of the 9th day (see Plate VIII. fig. 8) the 
abdominal air-sacs (s.a'"") have the appearance of oval cavities within 
somewhat conical outgrowths of the diaphragmatic mesoblast (d.a). 
At the beginning of the 10th day, when the separation of the 
pleural and peritoneal cavities is about completed’, the abdominal 
air-sacs project to a considerable extent posteriorly into the peritoneal 
cavity (cf. figs. 11, 12, and 20, s.a'"’), while more anteriorly (cf. left 
side of fig. 11 and figs. 21 and 22) they lie within the oblique ab- 
dominal septum (y, y), which assumes its final form only in connexion 
with them. 
At the beginning of the 12th day (cf. figs. 14, 15, 27, and 28) 
we find that the abdominal air-sacs not only have increased in size 
and extended more into the septum referred to, separating its two 
layers of peritoneum, but that they have begun to, as it were, strip 
off the peritoneal covering of the body-wall by extending behind it ; 
and this process goes on till in the adult (cf. fig. 47) comparatively 
little of the peritoneal lining of the intestinal portion of the ccelome 
remains applied to the body-wall. 
The first beginnings of the “anterior-’’ and “ posterior-inter- 
mediate” air-sacs are not quite so easy to trace. The former is 
conspicuous in the latter half of the 8th day, and both can be made 
out on the 9th. (The anterior intermediate sacs are shown in fig. 9, 
s.a'.) 
At the beginning of the 10th day when, as stated above, the 
avian diaphragm forms a complete partition, one can, in stained 
sections, distinguish two layers (which do not, however, exactly corre- 
spond to the two diaphragmatic septa of the adult) (¢f. figs. 11, 12, 
and 24). Inthe region of the ribs the muscles (m/) of the future 
pulmonary aponeurosis, or ‘‘ costopulmonary muscles” of Huxley, are 
indicated, and from this region a darkly staining layer extends 
inwards to the middle line passing dorsal to the cesophagus (¢/. 
fig. 24, ap.p). This darkly staining layer together with the above 
mentioned developing muscles indicates the “pulmonary aponeu- 
by 
1 It must be remembered that the dates in the case of the development of the 
chick are not absolute indices. As is well known, the rate of development 
under artificial incubation may differ considerably from that under a hen, and 
also in the case of different eggs artificially incubated. I have seen the peri- 
toneal not closed off completely from the pleural cavities in a chick said to 
have been artificially incubated for 11 days; and this agrees with Uskow’s ob- 
servations (5, p. 205). He remarks that a connexion between the pleural and 
peritoneal cavities exists on the 12thday. Yet, in the specimen naturally in- 
cubated, 9 days 1 hour (beginning of the 10th day), of which longitudinal 
horizontal sections are shown in figs. 11-13, no such connexion was to be made 
out in a continuous series of sections. So far as the stages illustrating the 
development of the air-sacs go (8th—-12th days), I have taken as my standard a 
series of naturally incubated embryos. 
Proc. Zoou. Soc.—1889, No. XXXI, 31 
