558 



THE HEAD-CAVITIES. 



The mesoblast in Elasmobranch embryos becomes first split into 

 somatic and splanchnic layers in the region of the head; and between 

 these layers there are formed two cavities, one on each side, which 

 end in front opposite the blind anterior extremity of the alimentary 

 canal; and are continuous behind with the general body cavity (fig. 

 20 A, vp). I propose calling them the head-cavities. The cavities 

 of the two sides have no communication with each other. 



Coincidently with the formation of an outgrowth from the 

 throat to form the first visceral cleft, the head-cavity on each side 

 becomes divided into a section in front of the cleft and a section 

 behind the cleft ; and at a later period it becomes, owing to the 

 formation of a second cleft, divided into three sections: (1) a section 

 in front of the first or hyomandibular cleft ; (2) a section in the 

 hyoid arch between the hyomandibvilar cleft and the hyobranchial or 

 first branchial cleft ; (3) a section behind the first branchial cleft. 



The front section of the head-cavity grows forward, and soon 

 becomes divided, without the intervention of a visceral cleft, into an 



anterior and posterior division. The 

 anterior lies close to the eye, and in 

 front of the commencing mouth in- 

 volution. The posterior part lies com- 

 pletely within the mandibular arch. 



As the rudiments of the successive 

 visceral clefts are formed, the posterior 

 part of the head-cavity becomes divided 

 into successive sections, there being 

 one section for each arch. Thus the 

 whole head-cavity becomes on each 

 side divided into (1) a premandibular 

 section ; (2) a mandibular section {vide 

 fig. 29 A, pp); (3) a hyoid section; 

 (4) sections in each of the branchial 

 arches. 



The first of these divisions forms 

 a space of a considerable size, with 

 epithelial walls of somewhat short 

 columnar cells Cfig. 381, Ipp). It is 

 situated close to the eye, and presents 

 a rounded or sometimes a triangular 

 figure in section. The two halves of 

 the cavity are prolonged ventralwards, 

 and meet below the base of the fore- 

 brain. The connection between them 

 appears to last for a considerable 

 time. These two cavities are the only 

 parts of the body cavity within the 

 head which unite ventrally. The sec- 

 tion of the head-cavity just described 



Fig. 381. Transverse section 

 through the front part of the 



HEAD OF A YOUNG PrISTIURUS EMBRYO. 



The section, owing to the cranial 

 flexure, cuts hoth the fore- and the 

 hind-brain. It shews the preman- 

 dibular and mandibular head cavities 

 Ipp and Ipp, etc. The section is 

 moreover somewhat obUque from side 

 to side. 



fh. fore-brain ; I. lens of eye ; m. 

 mouth ; pt. upper end of mouth, 

 forming pituitary involution ; lao. 

 mandibular aortic arch ; 1pp. and 

 2pp. first and second head cavities ; 

 \vc. first visceral cleft ; V. fifth 

 nerve ; aun. auditory nerve ; VII. 

 seventh nerve ; aa . dorsal aorta ; 

 acv. anterior cardinal vein; ch. 

 notochord. 



