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 1 0th 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 (c/".figs. 11, 12, and 20, s.a"), while more anteriorly (c/". 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 

 vyith them. 



At the beginning of the 12th day (c/. 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 

 laj'ers 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 coelome 

 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) {cf. figs. 11, 12, 

 and 24). In the region of the ribs the muscles {ml) 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 oesophagus {ef. 

 fig. 24, ap.p). This darkly staining layer together with the above 

 mentioned developing muscles indicates the "pulmonary aponeu- 



^ 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 maj' 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 12th day. 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 wag 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. 



Phoc. Zool. Soc— 1889, No. XXXI. 31 



