1892.] BODY-CAVITY IN SNAKES. 483 



water-snakes Hydrophis and Pelamis), one possible source of error 

 is eliminated, for in these there is no space round the fat, such as is 

 present in the great majority of Snakes. In Python the fat occurs 

 as a number of small separate lobules, quilted between membranous 

 tissue, and the condition is somewhat similar in Cylindrophis, though 

 anteriorly the fat-lobules tend to run together. 



On the other hand, in most Snakes the fat occurs as a continuous, 

 but often much folded band on either side, each of which hangs in 

 a well-marked lymph-cavity ' which might perhaps be taken, as it 

 has been taken in certain other Reptiles, for a part of the coelom 

 proper. Development, however, shows that this is not the case. The 

 fat and the space round it become differentiated at a comparatively 

 late embryonic stage, and the space probably arises in the same way 

 as, and should be placed in the same category with, the " cisterna 

 magna," in which runs the aorta ". This latter is, like the circum- 

 adiposal lymph-spaces, well developed in Snakes. Besides these 

 there may be a more or less distinct lymph-space round the kidneys. 

 It would have been impossible, in discussing the body-cavity 

 proper of Snakes, to omit a reference to these extra-^mtone&X lymph- 

 spaces, for they are certain to strike the observer, and may be in 

 some cases more conspicuous than the peritoneal cavity itself ; and 

 he might possibly take them for part of this and wonder why no 

 reference had been made to tliem. For further remarks on these 

 spaces, and figures showing their relations in adult Tropidonotus and 

 Vipera and advanced embryo of Tropidonotus, see paper " On the 

 Relations of the Fat-bodies of the Sauropsida." ' 



After opening the ventral body-wall of the Snake as described 

 above, it will be best, before further dissecting, to ascertain the 

 position of the right and most anteriorly situated kidney. If, 

 then, we cut through the membrane ventrad of the fat a little 

 to the right side of the animal, we, as explained above, in nearly all 

 Snakes cut into the right circumadiposal space ; this can be 

 followed forwards and backwards, as a continuous space from one 

 end of the fat to tiie other. If we next, turning up the fat, cut 

 through the inner membranous wall of this space at a point just 

 anterior to the right kidney ' we shall have cut into : — 



§ IV. (i.). J'he Single Posterior Peritoneal Space. 

 This is described by Retzius, (1) p. 91, (2) p. 517, and Lataste 



1 In certain other Snakes we see a condition of things intermediate between 

 this and what obtains in Pythons — in fact, we have an interesting and possibly 

 suggestive series, which need not, however, be discussed here. 



- Thus in advanced Elaphis embryos (Plate XXVIII. figs. A, B) the allantoic 

 arteries run forwards from the aorta to the umbilical stalk in the posterior part 

 of these circumadiposal lymph-spaces, as the aorta runs in the cisterna magna, 

 but there does not appear to be any communication between these spaces. 



3 Proc. Zool. Soc. London, 1889, plate hx. figs. 8, 9, 10. [N.B. g in fig. 9 

 and re in figs. 5 and 6 should be c.w.'\ 



* If we cut at random we shall possibly miss the peritoneal cavity alto- 

 gether, and may perhaps cut either into the " cisterna magna," or a lymph- 

 space that may surround the kidney. 



