THE ENDOSKELETON 687 



chondrocranium, while the membranous portion is called a membrano- 

 cranium. 



On each side of the notochord (which in the embryo extends as far 

 forward as the infundibulum of the brain) a horizontal plate of cartilage 

 is formed. These plates are known as parachordal plates (Figs. 310, 

 353). These extend laterally to the ears, forward to the end of the noto- 

 chord, and backward to the exit of the tenth nerve. The otic capsule 

 (cartilaginous) then grows about each internal ear and joins the para- 

 chordals. This forms a sort of trough in which the most caudal portion 

 of the brain lies. The floor of this trough is known as the basilar plate, 

 being formed of the parachordals and notochord as a floor, while the 

 sense capsules constitute the sides. 



From this basilar plate two cartilages pass forward on each side 

 forming a similar trough for the anterior part of the brain. According 

 to Professor Kingsley, the lower of these, called the trabeculae cranii, 

 "join the anterior margin of the basal plate, while the dorsal bars, the 

 alae temporales or alisphenoid cartilages are eventually connected with 

 the anterior wall of the otic capsules. In most vertebrates the trabecu- 

 lae and alisphenoids develop as a continuum, but in some elasmobranchs 

 they are at first distinct. The two trabeculae unite in front to form a 

 median ethmoid plate beneath the olfactory lobes of the brain, beyond 

 which they diverge as two horns, the cornua trabeculae, ventral to the 

 nasal organs. The floor of the trough in front of the ears is formed by 

 the ethmoid plate anteriorly, while behind, it is usually of membrane, 

 but in the elasmobranchs, cartilage gradually extends from one trabec- 

 ula to the other, closing last below the infundibulum and hypophysis, 

 these lying for a time in an opening (fenestra, later fossa hypophyseos), 

 and after the closure, in a pocket in the floor of the chondrocranium, one 

 of the cranial landmarks, the sella turcica." 



"In the more primitive vertebrates the trough is converted into a 

 tube around the brain by the extension of cartilages between the 

 alisphenoid cartilages and the otic capsules of the two sides dorsal to the 

 brain. This roof or tegmen cranii is usually incomplete, having one or 

 more gaps or fontanelles, closed only by membrane. In the higher ver- 

 tebrates the cartilage roof is at most restricted to a mere arch, the synotic 

 tectum, between the otic capsules of the two sides. 



"Later a pair of nasal capsules develop around the olfactory organs. 

 These are usually fenestrated and become united to the cornua, alisphe- 

 noids and ethmoid plate. In a similar way a sclera (sclerotic coat) forms 

 around each eye, but since the eye must move, this sense capsule 

 never unites with the rest of the cranium. Behind the otic capsules 

 a varying number of (four in some sharks and most teleosts ; in others 

 three; in amphibia two), occipital vertebrae are developed, which later 

 fuse with the rest of the chondrocranium. They alternate with myotomes 

 and nerves in this region as do the vertebrae of the vertebral column. 



