490 MR. G. W. BUTLER ON THE [June 14, 



§ VI. The Developmental History of the Pleuroperitoneal Cavity 

 of Snakes. 

 xlt 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 afl'ected by the coiling, and 

 though the modifications which produce the 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. 



§ YI. (i.). Early Embryos of Tropidonotus, Zamenis, and Vipera 

 (with gill-slits). 



For the earlier stages (about period II. of Rathke ^), I obtained a 

 series of embryos of Tropidonotus nati'ix, and a less complete one of 

 Zamenis geinonensis 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 ^ 



^ On the supposition that he desires to obtain a complete series of sections. 

 Doubtless much can be done without this, but iu 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. 



' Angtiis fragilis, Ckalcides mionecton, and apparently Acontias monodactyla. 

 However, Acontias ineleagris presents the condition of things that is usual in 



