68 



NERVOUS SYSTEM OF VERTEBRATES. 



which connected this mouth with the present pharyngeal region 

 (paleopharynx) were one or two gill clefts and the present mouth 

 has been formed from the second or third pair of ancestral gill 

 clefts. Behind the mouth a variable number of clefts are present 



^^lllnlll/ n-rrr 



n h St 



rrmrm mm 



n h St 



n h 



smmmrrnirnii 



St 



Fig. 37. — Diagrams representing the development of the buccal cavity, hypophy- 

 sis and nasal pit in Amphioxus and Petromyzon. After Legros. 



in existing vertebrates: in cyclostomes, 7 to 35; in selachians, 

 5 to 7; in amphibia and higher forms, 5 to 4. It is probable that 

 the number in primitive vertebrates was large. The number of 



