68 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



brain lies, its floor formed of parachordals and notochord (basilar 

 plate) and its sides of the sense capsules (fig. 67, 68). 



From this posterior part two cartilages extend forward on either 

 side, forming a somewhat similar trough for the anterior part of the 

 brain; the lower of these, the trabeciUae 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 (fig. 68). In most vertebrates the trabeculae 

 and alisphenoids develop as a continuum, but in some elasmobranchs 

 they are at first distinct (fig. 67). 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 comua tra- 

 beculae, ventral to the nasal organs. The floor of the trough in front 



Fig. 67. — Early chondrocranium of Acanthias, after Sewertzoff. (The brain in 

 outline.) als, alisphenoid cartilage; ch, anterior end of notochord; h, hyoid arch; ma, 

 mandibular arch, not yet divided into pterygoquadrate and Meckelian; oc, otic capsule; 

 /, trabecula; 1-5, branchial arches; cartilages dotted. 



of the ears is formed by the ethmoid plate anteriorly, while behind 

 it is usually of membrane, but in the elasmobranchs cartilage gradu- 

 ally extends from one trabecula to the other, closing last below the 

 infundibulum and hypophysis, these l5dng 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 elasmobranchs and amphibians the trabeculae are widely separated 

 until they reach the ethmoid plate, a condition correlated with the anterior 

 extension of the brain. This is the platybasic chondrocranium (fig. 68). In 

 the other classes the brain does not extend so far forward and the two trabeculae 

 meet just in front of the hypophysis (fig. 70) to continue forward as a trabecula 

 communis to the ethmoid region. The trabecula communis is usually com- 

 pressed between the eyes to a vertical interorbital septum. This represents the 

 tropibasic chondrocranium. 



