CHAP. X.] 



THE DEVELOPMENT OF THE CAT. 



323 



grows up around and over the embryo. This up-growth continues 

 until the ascending folds of the circumvallation meet and coalesce 

 ahove the embryo, and so form a completely closed sac above it. 

 This sac is the amnion (Fig. 148, -, '^ ,^). The cavity enclosed 

 by it is, of course, the same as the cavity of the " chamber" above 



mo 

 U, dl\ Li 



INITIAL STAGES OP DEVELOPMENT. 



Fig. 147. — Embryo seen in Transverse Section, showing formation of Somatopleitre and 

 Splanchnopleure, and of the Peritoneal Cavity. The part of the Somato- 

 pleurb which ascends beyond the embryo is not represented. 



mg. Medullary groove, 

 e. Ejiiblast. 

 m. Mesoblast. 

 74. Hypoblast. 

 Id. Lamiiiee dorsales. 

 Iv. Laminfe veutrales. 

 71. Notochord. 

 lie. Neural canal. 

 sp. Splauchnoiileure. 

 so. Somatopleure. 

 i. Intestine. 

 u. Umbilical vesicle. 



vfj. Prominences in which primitive urinary 

 and sexual glands arise. 



J). Peritoneal cavity. 

 me. Mesenterj'. 



A. Embryo, with the mesoblast as yet unsplit. 



B. Mesoblast split into somatopleure and 



splanchnopleure, and ■with an incipient 

 separation between the intestine and the 

 subjacent umbilical vesical. 



C. Further closing oil' of intestine from um- 



bilical vesicle. 



D. The body cavity (or peritoneal cavity) com- 



pletely enclosed by somatopleure, while the 

 part of the splanchnopleure between the 

 intestine and superior part of the embryo 

 has become attenuated into a mesentery (me). 



mentioned as being " gradually formed between the surface of the 

 embryo " and the " more and more bent over surface " of the 

 ascending circumvallation. The embryo lies in the amnion as in a 

 water-bed. For though at first this sac is very small, it subse- 

 quently enlarges and becomes filled with a certain fluid called the 

 liquor amnii. As the ascending folds are (as before said) double on 

 all sides (Fig. 148, ^) it follows that upon their coalescence a double 

 sac is at first formed. The amnion is the inner of these two sacs. 

 The sac external to it (formed of the outer layers of all the folds) 

 disappears by coalescing with the chorion, or outer envelope of the 

 ovum, within which of course all the changes here described take 

 place (Fig. 148, ■^). As the primitive peritoneal cavity was con- 

 tinuous with the space enclosed between the two layers of tfte 

 ascending folds, it follows that (upon the completion of the amnion) 

 there comes to be for a time a communication (over the back of the 

 embryo and above the amnion) between the two sides of the peri- 

 toneal cavity. This space is occupied by a more or less fluid or 

 gelatinous substance (Fig. 148, 5r). This communication is, how- 

 ever, soon cut off by the descent and in growth of that part of the 

 somatopleure which forms the side- walls of the body of the embryo. 



Y 2 



