672 



THE SKELETAL SYSTEM 



in many teleost fishes, the basic ventro-lateral regions of the cartilagi- 

 nous neurocranium only are formed (figs. 318A, B; 319A, B). This 

 basic chondrocranium undergoes considerable ossification, forming 

 cartilage bones, which replaces the cartilage of the chondrocranium. 

 These cartilage bones are supplemented by superficially developed 

 membrane bones which become closely associated with the cartilage 

 bones. The adult skulls of these vertebrates are highly ossified struc- 

 tures, composed of cartilage and membrane bones. (See Tables 2 and 

 3.) A few cartilaginous areas persist in the adult skull, more in teleost 

 fishes than in the reptiles, birds, and mammals (Kingsley, '25 and De 

 Beer, '37). 



4) Vicissitudes of the Splanchnocranium. The early visceral skeleton, es- 

 tablished in the embryo, experiences many modifications in its development 

 in the diff'erent vertebrate groups. 



In the elasmobranch fishes, the first visceral (mandibular) arch on either 

 side gives origin to an upper jaw element, composed of the palatoquadrate 

 (pterygoquadrate) cartilage, and a lower jaw element or Meckel's cartilage 



angulosple 



Fig. 317. Developmental stages of neurocranium in the frog. (A and B redrawn from 

 De Beer, "?7, after Pusey; C. redrawn and modified from Marshall, 1893, Vertebrate 

 Embryology, New York, Putnam's Sons.) (A) Intermediate condition between larval 

 and adult form. (B) Adult form of cartilaginous cranium, present after metamorphosis. 

 (C) Adult neurocranium composed of membrane and cartilage bones associated with 

 basic cartilaginous neurocranium (see Table 1). Cartilage = coarse stipple; bone = fine 

 stipple. 



