comua 

 trabeculae 



ethmoid plate 



olfactory 

 capsule 



THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 97 



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 

 (F lg . 31J). 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 comua trabeculae. The parachordals unite to form the basal plate which then fuses 

 on either side which 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 

 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 as fenestrae 

 (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 fonlanelles , closed by membranes. Diagrams of the 

 formation of the chondrocranium are given in Figure 31; also in W, page 144; Wd, page 76; 

 P and H, page 75; K, page 69. 



References on the skull are: K, pages 67-69; W, pages 142-46; Wd, pages 74-80; 

 P and H, pages 75-76; H, pages 455-56; L, pages n-13. 



Fig. 32. — Chondrocranium of a urodele larva. 

 (After Gaupp in Hertwig's Handbuck der ver- 

 gleichenden und experimentellen Entwickelungslehre 

 der Wirbeltiere.) 



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 



