830 DEVELOPMENT. [OH. LVIII. 



From the third visceral arch, the greater cornu and body of the 

 hyoid bone arise. In man and other mammalia the other arches 

 disappear. They occupy the position where the neck is afterwards 

 developed. 



A distinct connection is traceable between these visceral arches 

 and certain cranial nerves : the trigeminal, the facial, the glosso- 

 pharyngeal, and the vagus. The ophthalmic division of the 

 trigeminal supplies the fronto-nasal process ; the superior and 

 inferior maxillary divisions supply the maxillary and mandibular 

 arches respectively. 



The facial nerve distributes one branch (chorda tympani) to 

 the first visceral arch, and others to the second visceral arch. 

 Thus it divides, enclosing the cleft next behind the mouth. 



Similarly, the glosso-pharyngeal divides to enclose the third 

 visceral cleft, its lingual branch being distributed to the second, 

 and its pharyngeal branch to the third arch. 



The vagus, too, sends a branch (pharyngeal) along the next 

 arch, and in fishes gives off paired branches, which divide to 

 enclose the remaining branchial clefts. 



Development of the Vascular System. 



We have already mentioned that at an early stage in develop- 

 ment the area vasculosa makes its appearance in the pai't of the 

 yolk-sac which is separated from the body of the embryo by a 

 clear space (see fig. 632). This is produced by mesoblastic cells 

 becoming hollow, filled with blood, and uniting to form embryonic 

 capillaries (see p. 422). These vessels converge to two trunks, 

 one on each side, which are called the omphalo-mesenteric or 

 vitelline veins, and these lead to the embryonic heart. 



The heart is developed in the splanchnopleur, by a folding off 

 of the pleuroperitoneal cavity. It appears beneath the posterior 

 end of the fore-gut. Its first appearance, however, is as two 

 tubes, one on each side of the fore-gut. This is shown in outline 

 in the next diagram (fig. 646). 



It will be seen that the medullary groove is enlarged anteriorly, 

 and the primary optic vesicles are growing from the first cerebral 

 enlargement. Eight pairs of protovertebrae are shown ; and on 

 either side of the head the primitive tubular heart (H) is seen. 



If we look at a rather later stage, shown in transverse section 

 in the next figure (fig. 647), we see the two tubular hearts cut 

 across, and approaching one another beneath the alimentary 

 canal, which is being cut off from the yolk-sac. 



