DEVELOPMENT OF ELASMOBRANCH FISHES. 109 



into (1) An epitlieliura on the free surface ; from this are 

 derived the glandular parts of the kidneys and functional parts 

 of the genital glands; and (2) a subjacent stroma which forms 

 the basis for the connective tissue and vascular parts of these 

 organs. 



To the histor}^ 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 

 alimentary canal. 



Body- Cavity and parietal plates. By the close of stage H, as 

 has beeft 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. x. 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 (PL X. fig. 1). At the caudal swelling 

 the mesoblast plates cease to be distinctly divided into soaiato- 

 pleure and splanchnopleure, and more or less fuse with the 

 hypoblast of the caudal vesicle (PI. x. 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 everywhere, is formed of two independent lateral 

 halves (PL xi. fig. 9 a). Anteriorly, opposite the hind end of 

 the small intestine, these two lateral halves unite ventrallv to 



