SKULL — ^CHONDROCRANIUM 



59 



Vertebrate scale, the lowest sharks having eight (not including the mouth) 

 while in higher Vertebrates the number is five or six. This variation in number 

 may be explained in two ways. In the higher groups there has been a loss at 

 the posterior end of the series; or in all Vertebrates there is a zone of vegetative 

 growth at the junction of head and trunk, and the amount of growth at this 

 point has been more and more Hmited as the phylum has developed. Certain 

 it is that the two preotic arches (mandibular and hyoid) belong to the head, 

 and as the relations of these are similar in most respects to the arches behind the 

 ear, probably all arches belong to the head. 



Chondrocranium 



In the following outline of the development and structure of the 

 skull, only those features are included which are common to all 

 Gnathostomes, the Cyclostome skull 

 being considered separately (p. 78). 

 The simplest picture, so far as cartilage 

 parts (chondrocranium) are concerned 

 is furnished by Elasmobranchs which 

 are here used as a basis. But since 

 the head of a shark is strongly flexed 

 in the early embryo before skeletal 

 parts appear (fig. 86), the early skull is 

 described here as if extended in a 

 straight line with the notochord, a 

 condition largely regained in the adult. 



In the embryo (fig. 65) the noto- 

 chord extends into the head as far as 

 the infundibulum and its associated 

 ectodermal structure, the hypophysis. 

 On the supposition that vertebrae are 

 only formed around the chorda, only 

 those parts of the skull behind the 

 hypophysis can be vertebral, but 

 whether there be any vertebrae as far 

 forwards as this has not been shown. 

 This limitation of the tip of the 

 notochord allows the cranium to be 

 divided into chordal and prechordal parts. 



In the chordal part a plate of (parachordal) cartilage arises on 

 either side of the notochord, extending as far laterally as the tissue 



Fig. 65. — Early chondrocranium 

 of an Elasmobranch, straightened; 

 compare with fig. 86. 0/5-, spheno- 

 lateral; ctr, trabecular cornua; ep, 

 ethmoid plate; fhyo, hypophysial 

 fenestra; oc, otic capsule; ov, occipital 

 vertebrae; n, notochord; pc, parachor- 

 dals; tr, trabeculse. 



