363 f. M. BAtFOtJR. 



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 Pleuro-peritoneal Cavity into the 



Head. 



In the earlier part of this paper (p. 346) I called attention 

 to the extension of the separation between somatopleure and 

 splanchuopleure into the head, forming a space continuous 

 with the pleuro-peritoneal cavity (PI. XIV, fig. 8 a, jo p') ; this 

 becomes 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 appearance 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 necessarily 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 bifur- 

 cates. 



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

 fig. \4!,pp'. They are in reality somewhat flattened spaces, 

 lined by a mesoblastic epithelium, the epithelium on the 

 inner surface of the space corresponding to the splanchuo- 

 pleure, 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 showing the fundamental similarity between 

 the head and the remainder of the body. 



The Pituitary Body. 



All my sections seem to prove that it is a portion of the 

 epiblastic involution to form the mouth which is pinched oft' 

 to form the pituitary body, and not a portion of the hypo- 

 blast of the throat. Since Gotte ('Archiv. fiir Micr. Anat./ 



