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Show 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 VIIL 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 completedl, 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 abdominal 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-intermediate " 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 9 th. (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 correspond 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 (cf. fig. 24, ap.p). This darkly staining layer together with the above mentioned developing muscles indicates the "pulmonary aponeu- 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 m a y differ considerably from that under a hen, and also in the case of different eggs artificially incubated. I have seen the peritoneal 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 observations (5, p. 205). H e remarks that a connexion between the pleural and peritoneal cavities exists on the 12th day. Yet, in the specimen naturally incubated, 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 m y standard a series of naturally incubated embryos. PROC ZOOL. Soc-1889, No. XXXI. 31 |
