STAGES G TO K. THE BODY-CAVITY. 327 



basis for the connective-tissue and vascular parts of these 



organs. 



To the history of these parts a special section is devoted ; 

 and I now pass to the description of the mesoblast which lines 

 the body-cavity and forms the connective tissue of the body- wall, 

 and the muscular and connective tissue of the wall of the alimen- 

 tary canal. 



Body-cavity and Parietal Plates. By the close of stage H, as 

 has been already mentioned, a cavity is formed between the 

 somatopleure and splanchnopleure in the anterior part of the 

 trunk, which rapidly widens during the succeeding stages. 

 Anteriorly, it invests the heart, which arises during stage G, 

 as a simple space between the ventral wall of the throat 

 and the splanchnopleure (PI. 11, fig. 4). Posteriorly it ends 

 blindly. 



This cavity forms in the region of the heart the rudiment of 

 the pericardial cavity. The remainder of the cavity forms the 

 true body-cavity. 



Immediately behind the heart the alimentary canal is still 

 open to the yolk-sac, and here naturally the two lateral halves of 

 the body-cavity are separated from each other. In the tail of 

 the embryo no body-cavity has appeared by stage I, although 

 the parietal plates of mesoblast are distinctly divided into somatic 

 and splanchnic layers. In the caudal region the lateral plates of 

 mesoblast of the two sides do not unite ventrally, but are, on the 

 contrary, quite disconnected. Their ventral edge is moreover 

 much swollen (PI. n, fig. i). At the caudal swelling the meso- 

 blast plates cease to be distinctly divided into somatopleure and 

 splanchnopleure, and more or less fuse with the hypoblast of the 

 caudal vesicle (PL 11, fig. 2). 



Between stages I and K the body-cavity extends backwards 

 behind the point where the anus is about to appear, though it 

 never reaches quite to the extreme end of the tail. The backward 

 extension of the body-cavity, as is primitively the case every- 

 where, is formed of two independent lateral halves (PI. 1 1, fig. 9^). 

 Anteriorly, opposite the hind end of the small intestine, these 

 two lateral halves unite ventrally to form a single cavity in which 

 hangs the small intestine (PI. 11, fig. 8) suspended by a very 

 short mesentery. 



