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Show 1892.] BODY-CAVITY IN SNAKES. 491 does not extend forwards into the region of the lung l; it is confined to the hepatic region-the pancreas, which forms the posterior wall of the space, being, together with the gall-bladder, at this stage and for some time longer in contact with the liver. But as development proceeds, and as the lung extends back to and beyond the hinder end of the liver, and as the pancreas and gall-bladder come to lie, as they almost invariably do in Snakes, a considerable distance behind the liver2, this space (if not obliterated) comes to be, with rare exceptions, entirely posthepatic in position. § VI. (ii.). Embrgos of Elaphis quadrilineatus, 11 cm. long. The next stage that I have, an Elaphis embryo 11 cm. long (Plate XXVIII. fig. A ) , (all allowance being made for Elaphis being a larger Snake than Tropidonotus or Zamenis, with larger eggs), is considerably more advanced than the stage just described, and yet for our present purpose there is no important gap between them. There is in fact, as far as the pleuroperitoneal cavity is concerned, at first sight as yet nothing to suggest the characteristic Ophidian condition. The liver-lobe of either side and the lung for the greater part of its length project freely into the common pleuroperitoneal cavity. The only definite change that we have to note is the closing of the " Foramen of Winslow " ; this, however, not only occurs in Birds (Gallus), Crocodiles, and many Chelonians, but also in the snake-like but truly lacertilian Amphisbaenidse. In fact an Elaphis embryo of 11 cm. long is still lacertilian as to its pleuroperitoneal cavity ; but, nevertheless, the changes that are shortly to supervene are foreshadowed. Lizards, and perhaps little importance is to be attached to tbe difference. Whichever is the more primitive state of things, the one may easily be derived from the other. The condition in Snakes and the Scincoids first mentioned is probably associated with the elongation of form, and with the origin of the liver at some distance behind the point of origin of the lungs. In fact, in these Lizards, as in all the Snakes I have examined (with the exception of the species of Vipera, Hydrophis, Pelamis, and less markedly of Typhlops), there is even in the adult a distinct gap between the anterior end of the liver and the heart. 1 Some of the Snakes examined (see list, p. 481), viz. the Pythonidaa (Eryx, Enygrus, Python) and Xenopeltis, have two well-developed lungs, the right, however, being the larger. Others, viz. Rhinophis, Cylindrophis, Aspidura, Elaps, have a more or less distinct rudiment of a left lung. Others again- Tropidonotus, Elaphis, Bipsas-have the merest trace of this, only to be found by careful search near the posterior corner of the heart. In some, again, Vipera (berus and asp>is), Crotalus, Lamprophis, and others, I did not find any trace of a left lung. In Vipera aspis I find no trace of a left lung even in early embryos. Though I have no embryonic stages of the Pythonidse or Xenopeltidse, a comparison of their anatomy with that of the more usual one-lunged forms seems to assure us that, for our present purpose, there is no noteworthy difference between them. The left lung, when present, lies between the dor so-lateral wall of the left liver-sac and the oesophagus, in a position, in fact, corresponding to that of the right lung of the other side, and has not, any more than its fellow, any trace of pleural cavity round it. 2 However, in Rhinophis and certain specimens of Aspidura the gall-bladder is close to the liver, and it is not far removed in the C o m m o n Viper (Vipera berus). |
