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Show 490 MR. G. W. BUTLER O N T H E [June 14, § VI. The Developmental History of the Pleuroperitoneal Cavity of Snakes. At first sight the aspect of a Snake embryo is perhaps forbidding to the embryologist \ During much of its early existence great part of such a Snake is coiled round its allantoic stalk in such a way that it cannot be uncoiled, and one may have to do with the same embryo cut through nine times in one section. On the other hand, in later stages, when the embryo can be straightened out, it is apt to be desperately long. However, the part of the animal which chiefly concerns us in the earlier stages is not affected by the coiling, and though the modifications which produce tbe characteristic relations of the peritoneum of the adult Snake only arise at a comparatively late embryonic stage, one comes to the end of even a six-inch Snake sooner than might be expected, especially when, as in the present case, it is not necessary that the sections should be very thin. § VI. (i.). Early Embryos of Tropidonotus, Zamenis, and Vipera (with gill-slits). For the earlier stages (about period II. of Rathke a ) , I obtained a series of embryos of Tropidonotus natrix, and a less complete one of Zamenis gemonensis and Vipera aspis. These stages extend from (i) a time, soon after the first appearance of the allantois, when there were traces of but one or two postoral clefts and the spiral coiling had not begun, to (ii) a time when there were 4 complete coils in the abdomino-caudal region, and when, though the gills were hardly so apparent as in a stage with only 3 coils, sections showed that there were here, as in that stage, 4 pairs of postoral gill-pouches, the first two of which communicated with the exterior. In the most advanced of these earlier stages the pleuroperitoneal cavity presents a condition of things similar to that which we find in Lizards. That is to say, besides the main pleuroperitoneal cavity continuous throughout its whole extent, we have to the right of the stomach a " lesser peritoneal " or " omental " cavity, communicating with the right half of the main pleuroperitoneal one by a "Foramen of Winslow." The omental sac proper is, however, very small. Its anterior recess ["Recessus superior sacci omenti" of His-my " pulmo-hepatic recess " (5)], which, in Birds, Crocodiles, Chelonia, and most Lizards, runs forwards between, and is bounded by, the oesophagus and the lung and liver-lobe of the right side, and their connecting ligaments), in these Snake embryos, as in certain Scincoid Lizards \ 1 On the supposition that he desires to obtain a complete series of sections. Doubtless much can be done without this, but in dealing with a subject like that before us, when microscopic spaces have to be traced and it is often desirable to be able to prove a negative-to prove, for instance, that two small spaces do not communicate-no other method is equally satisfactory. 2 Eathke, ' Entwick. d. Natter,' Konigsberg, 1839. 3 Anguis fragHis, Chalcides mionecton, and apparently Acontias monodactyla. However, Acontias meleagris presents the condition of things that is usual in |