THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 



97 



cornua 

 trabeculae 



ethmoid plate 



these are designated as the parachordal cartilages. Another pair is laid down immediately 

 anterior to the parachordals; these are the prechordal cartilages or trabeculae. Cartilaginous 

 capsules also develop around each of the paired sense organs of the head olfactory sacs, 

 eyes, and ears. These are known respectively as the nasal capsules around the olfactory- 

 sacs, the optic capsules around the eyes, and the otic capsules around the internal ears 

 (Fig. 31-4). Fusion of these five paired cartilages then occurs. The anterior ends of 

 the prechordals fuse together to form the ethmoid plate which in turn fuses laterally with the 

 nasal capsules. The prechordals continue in front of the ethmoid plate as two projections, the 

 horns or cornua trabeculae. The parachordals unite to form the basal plate which then fuses 

 on either side with the otic capsules. The posterior ends of the prechordals unite with the 

 basal plate (Fig. 31 B). The optic capsules 

 never become fused to the skull since it is 

 necessary that the eyes remain independ- 

 ently movable; the optic capsules persist 

 around the eye as a tough protective coat, 

 the sclera. As a result of the fusion of 

 prechordals, nasal capsules, parachordals, 

 and otic capsules, a nearly continuous plate 

 of cartilage is formed below the ventral 

 side of the brain and inclosing the sense 

 organs. In addition, it is highly probable 

 that certain vertebrae are fused to the 

 posterior end of the basal plate. The 

 cartilaginous structure arising in this way 

 is named the chondrocranium. It occurs as 

 a stage in the development of the skull in 

 the embryos of all vertebrates. It must 

 be emphasized that in the majority of 

 vertebrate embryos the chondrocranium 



occipital 

 condyle 



notochord 



FIG. 32. Chondrocranium of a urodele larva. 

 (After Gaupp in Hertwig's Handbuch der ver- 

 gleichenden und experimentellen Entwickelungskhre 

 der Wirbeltiere.) 



has the form of an elongated, somewhat 

 curved plate ventral to the brain and sup- 

 porting the brain on its concave dorsal 

 surface. Generally it has neither lateral 

 nor dorsal walls except at the extreme pos- 

 terior end where it gives rise to a narrow 



dorsal arch, the synotic tectum. Usually there are gaps in this plate, known zsfenestrac 

 (Fig. 32). In the elasmobranch and other fishes the chondrocranium gradually extends dorsally 

 and finally incloses the brain in a cartilaginous box. The roof of this is generally incomplete, 

 presenting one or more gaps, the fontanelles, closed by membranes. Diagrams of the 



formation of the chondrocranium are given in Figure 31; similar figures will also be found in 



W, Wd, P and H, and K. 



For more complete accounts of the development and comparative anatomy of the skull, 



K, W, and Wd should be consulted. 



2. The chondrocranium of the dogfish. Study the chondrocranium of the 

 dogfish preserved in jars. It is a cartilaginous mass inclosing the brain and 

 without sutures or divisions. The dorsal surface is broad and flat while the 

 ventral surface is narrower and more irregular. The anterior end of the chondro- 

 cranium is drawn out into a troughlike structure, the rostrum. Each side of 



