SKULL 



577 



pituitary body (Fig. 152 C). The cranial cartilage does not 

 become segmented, but gives rise to a pair of horizontal 

 bars, \hQ parachordals (FE) : these are continued forwards, 

 diverging around the pituitary body, as the trabecules cranii 

 {Tr), and thus a support is formed for the developing 

 brain. The two parachordals and trabeculte then unite 

 A 



J'jT 



Fig. 152. — A and B, two stages in the development of the chondrocranium. 

 A. eye; AF. antorbital process; B. basal plate, formed from the parachordals; 

 C. notochord ; Ct. anterior process of trabecula ; N^Nk. nose; O. ear; 01. 

 position of foramina for olfactory nerves ; PE. parachordal cartilage ; Pf . post- 

 orbital process; PR. pituitary space ; .S'. nasal septum; Tr. trabecula. (From 

 Wiedersheim's Anatomy.) 



respectively with one another, and so form a firm 

 floor (B) for the future brain-case, which is gradually 

 developed by the floor growing up on either side and 

 eventually meeting to a greater or less extent above the 

 brain : there is never, however, a complete cartilaginous roof 

 to the cranium, parts of which are only membranous and 

 form the fontanelles (pp. 43 and 420). In the meantime 



Pract. Zool. P P 



