DEVELOPMENT OF THE ENDOSKELETON 



669 



MOUT^ 

 MECKEL'S CARTILAGE 



OTIC CAPSULE 



<7V^YOI0 

 K-\ ARCH 



^ A. 



■PHARYNGOBRANCHIAL 

 EPIBRANCHIAL 

 CERATOBRANCHIAL 

 BRANCHIOSTEGAL 



PTERYGOQUAORATE CARTILAGE 

 ORBIT NEUROCRANIUM 



CKEL'S \ 

 !TILAGE \ 



HYPOBRANCHIAL 

 YOMANDIBULAR CARTILAGE 



BASIBRANCHIAL 



SPLANCHNOCRANIUM 



Fig. 315. Developmental stages of the chondrocranium in the dogfish, Squalus 

 acanthias. (A and B redrawn from EI-Toubi, '49. Jour. Morph., 84.) (A) Early de- 

 velopmental stage, 37-mm. embryo, lateral view. (B) Intermediate stage, 45-mm. 

 embryo, lateral view. (C) Branchiostegal (gill support) rays attached to ceratobranchial 

 segment of gill arch. (D) Adult stage of chondrocranium (neurocranium plus splanch- 

 nocranium), lateral view. 



1) Neurocranium or Cranium Proper. The neurocranium is present in 

 three main forms in the vertebrate group: 



( 1 ) a complete cartilaginous cranium without dermal reinforcing bones, 

 as in cyclostomatous and elasmobranch fishes (fig. 315D), 



(2) an inner cartilaginous cranium, associated with an outer or surround- 

 ing layer of bony plates, as in Amia (fig. 316C, D), the adult skull 

 of Necturus and the frog being similar but slightly more ossified (fig. 

 317B, C), and 



(3) an almost entirely ossified cranium, in teleosts, reptiles, birds, and 

 mammals (figs. 318C; 319C, D, E). 



Various degrees of intermediate conditions exist between the above 

 groupings. 



2) Visceral Skeleton or Splanchnocranium. The splanchnocranium or vis- 

 ceral skeleton consists, of a number of cartilaginous or bony arches which tend 

 to enclose the anterior portion of the digestive tube (fig. 315D). They are 

 present in pairs, one arch on one side, the other arch on the other side. The 

 first two pairs are related to the skull in gnathostomes. The succeeding pairs 

 of visceral arches are associated with the branchial or gill apparatus in fishes 

 and in certain amphibia, such as Necturus. 



3) Development of the Skull or Neurocranium. The neurocranium of all 

 vertebrates from the fishes to the mammals possesses a beginning cranial con- 



