THE MUSCULAR SYSTEM. 



677 



vn~. 



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

 section in the hyoid arch between the hyomandibular 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 involution. The posterior 

 part lies completely within the man- 

 dibular arch. 



As the rudiments of the succes- 

 sive visceral clefts are formed, the 

 posterior part of the head-cavity be- 

 comes divided into successive sec- 

 tions, there being one section for 

 each arch. Thus the whole head- 

 cavity becomes on each side divided 

 into (i) 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 (fig. 381, ipp}. It is 

 situated close to the eye, and pre- 

 sents a rounded or sometimes a 

 triangular figure in section. The 

 two halves of the cavity are pro- 

 longed 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 section of the head-cavity 

 just described is so similar to the remaining sections that it 

 must be considered as serially homologous with them. 



The next division of the head-cavity, which from its position 



FIG. 381. TRANSVERSE SEC- 

 TION THROUGH THE FRONT PART 

 OF THE HEAD OF A YOUNG PRIS- 

 TIURUS EMBRYO. 



The section, owing to the cra- 

 nial flexure, cuts both the fore- 

 and the hind-brain. It shews the 

 premandibular and mandibular 

 head-cavities ipp and ipp, etc. 

 The section is moreover somewhat 

 oblique from side to side. 



fb. fore-brain ; /. lens of eye ; 

 m. mouth ; pt. upper end of mouth, 

 forming pituitary involution; lao. 

 mandibular aortic arch; ipp. and 

 ipp. first and second head-cavities; 

 \vc. first visceral cleft; V. fifth 

 nerve ; aim. auditory nerve ; VII. 

 seventh nerve ; aa. dorsal aorta ; 

 acv. anterior cardinal vein ; ch, 

 notochord. 



