OCR Text |
Show 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 1 which might perhaps be taken, as it has been taken in certain other Reptiles, for a part of the ccelom 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 2. 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-peritoneal 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 them. For further remarks on these spaces, and figures' showing their relations in adult Tropidonotus and Vipera and advanced embryo of Tropidonotus, see paper " O n the Relations of the Fat-bodies of the Sauropsida."3 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 the 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 4 we shall have cut into :- § IV. (i.). The 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. 2 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 lix. figs. 8, 9, 10. [N.B. g in fig. 9 and re in figs. 5 and 6 should be c.w.] 4 If we cut at random we shall possibly miss the peritoneal cavity altogether, and may perhaps cut either into the " cisterna magna," or a lymph-space that may surround the kidney. |