189 2. J 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 pro- 

 portionate to the rest of the body. 



Turning now to the condition of the body-cavity, we find that in 

 the Elaphis embryo, 1.5 cm. long, the lung has become entirely 

 excluded from that cavity, or, rather, that part of the body-cavity 

 which in the 1 1 cm. stage extended round the outer and dorsal 

 walls of the lung has been entirely obliterated (figs. 2^, 3^). 

 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. 1"^) ; and, judging by the relations of the poste- 

 rior end of the lung at that time (fig. 3^), it is but natural to conclude 

 that the great length of lung which now extends behind the liver 

 (cf. figs. JB and 3^) has developed where we find it, by burrowing 

 backwards as it were in the fibrous tissue dorsad of the peritoneal 

 cavity. We 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^ and 2^). 



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. 2\ **') ; 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. 1^ and 3^), 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. 2^, * ') 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. 2^, P.l-liver-sac). 

 The closing of this sac has resulted, firstly from an extension back- 

 wards 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 



