OCR Text |
Show 1892.] BODY-CAVITY IN SNAKES. 493 pancreas have become separated from the liver, and that the lung has grown a considerable distance backwards. In fact, an embryo of this stage, but for its comparative stoutness and the persistence of the Wolffian bodies in front of the kidneys, is very similar to the adult in the proportion of its parts and the position of the viscera. It is curious that the liver does not seem to have grown in length proportionate to the rest of the body. Turning now to the condition of the body-cavity, we find that in the Elaphis embryo, 15 cm. long, the lung has become entirely excluded from that cavity, or, rather, that part of the body-cavity which in the 11 cm. stage extended round the outer and dorsal walls of the lung has been entirely obliterated (figs. 2B, 3B). Remembering, however, that in the 11 cm. stage it was only the part of the lung in the region of the liver (at that time the greater part of the lung) which projected freely into the body-cavity, it will not surprise us so much as it might otherwise do to find that no part of the lung is now surrounded by that space. It will be remembered that, anterior to the liver, the pleural portion of the ccelom was in the preceding stage already in great part obliterated (fig. 1A) ; and, judging by the relations of the posterior end of the lung at that time (fig. 3 A), it is but natural to conclude that the great length of lung which now extends behind the liver (cf. figs. B and 3B) has developed where we find it, by burrowing backwards as it were in the fibrous tissue dorsad of the peritoneal cavity. W e have, then, only to account for the exclusion from the body-cavity of that part of the lung which lies in the hepatic region (compare figs. 2 A and 2B). Now, in the 11 cm. stage there was as described a considerable development of fibrous connective tissue, both on the ventral and dorsal free surfaces of the lung (fig. 2A, * * ' ) ; and the idea naturally suggests itself that, as far as that part of the lung which lies in the region of the liver is concerned, the fibrous tissue ventral to the lung [working backwards and forwards from the points opposite the anterior and posterior ends of the liver (cf figs. 1A and 3 A), where we saw the lung in the 11 cm. stage excluded from the peritoneal cavity] has formed a " diaphragm," similar in its relation to the lung, though perhaps not otherwise homologous, to the " diaphragm " of Birds; and that almost synchronously the fibrous tissue dorsad of the lung (fig. 2A, *') has obliterated the pleural cavity, thus produced, as the pleural cavity is obliterated in Birds. But of course, in the absence of an intermediate stage, one cannot be absolutely certain of what happens ; and it is possible that the changes which have taken place may be in part comparable to those which lead to the formation of the " diaphragm " in Mammals. With regard to the other divisions of the body-cavity, the left half of the liver now lies in a closed sac (the left liver-sac) (fig. 2B, P.l-liver-sac). The closing of this sac has resulted, firstly from an extension backwards of that obliteration of the peritoneal space on the left of the oesophagus which was seen taking place in the previous stage, and secondly from the connection with the lateral body-wall of the fibrous PROC. ZOOL. Soc-1892, No. XXXIV. 34 |