DEVELOPMENT OF THE ELASMOBRANCH FISHES. 105 



The Head. 



The study of the development of the parts of the head, on 

 account of the crowding of organs which occurs there, always 

 presents greater difficulties to the investigator than that_of the 

 remainder of the body. My observations upon it are cor- 

 respondingly incomplete. I have, however, made out a few 

 points connected with it in reference to some less well-known 

 organs, which I have thought it worth while calling attention to 

 in this preliminary account. 



The continuation of the Plenro-peritoneal Cavity into the 



Head. 



In the earlier part of this paper (p. 86) I called attention 

 to the extension of the separation between somatopleure and 

 splanchnopleure into the head, forming a space continuous with 

 the pleuro-peritoneal cavity (PI. 3, fig. 8 a, pp'} ; this be- 

 comes more marked in the next stage, and, indeed, the pleuro- 

 peritoneal cavity is present for a considerable time in the head 

 before it becomes visible elsewhere. At the time of the appear- 

 ance of the second visceral cleft it has become for the most 

 part atrophied, but there persist two separated portions of it in 

 front of the first cleft, and also remnants of it less well marked 

 between and behind the two clefts. The visceral clefts neces- 

 sarily divide it into separate parts^ 



The two portions in front of the first visceral cleft remain 

 very conspicuous till the appearance of the external gills, and 

 above the hinder one of the two the fifth nerve bifurcates. 



These two are shewn as they appear in a surface view in 

 fig. 14, //. They are in reality somewhat flattened spaces, 

 lined by a mesoblastic epithelium ; the epithelium on the inner 

 surface of the space corresponding to the splanchnopleure, and 

 that on the outer to the somatopleure. 



I have not followed the history of these later than the time 

 of the appearance of the external gills. 



The presence of the pleuro-peritoneal cavity in the head is 

 interesting, as shewing the fundamental similarity between the 

 head and the remainder of the body. 



