60 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



cartilaginous and osseous, and in the early stages there is no trace of seg- 

 mentation or of vertebrae, the Okenian segments only appearing with 

 the appearance of bone. The skull may be divided into two portions, 

 a cranium, composed of a case for the brain, and sense capsules en- 

 closing the organs of special sense (ears, eyes and nose) ; and a visceral 

 skeleton, more or less intimately related to the anterior end of the 

 digestive tract. 



Development of the Skull. 



Little is known in detail of the development of the membranous 

 skull save that it envelops the brain and sense organs, extends into 

 the visceral region, and that it affords the substance in which the second, 

 or cartilaginous, skull is formed. 



FIG. 59. 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; t, trabecula; 

 1-5, branchial arches. 



The cartilaginous envelope of the brain and sense organs is called 

 the chondrocranium. The notochord extends forward beneath 

 the brain as far as the infundibulum and a horizontal cartilage plate 

 forms on either side of it. These parachordal plates extend later- 

 ally as far as the ears, forward as far as the end of the notochord and 

 back to the exit of the tenth nerve. A little later a cartilaginous otic 

 capsule forms around each ear and joins the parachordals, thus form- 

 ing a trough in which the posterior part of the brain lies, its floor formed 

 of parachordals and notochord (basilar plate) and its sides of the 

 sense capsules. 



From this posterior part two cartilages extend forward on either 

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



