SKELETON 67 



theriomorphs, pythonomorphs, ichthyosaurs, and plesiosaurs, and 

 possibly the entoplastron of the chelonians (fig. :^8, p. 46) is the 

 same structure. It has not been recognized in birds, but it appears 

 in the monotremes among mammals (fig. 124), with nearly the same 

 relations as in the lacertilians. ^ . 



The Skull Ty 



The skull is a complex structure and the last word concerning its 



j composition has yet to be said. A century ago Oken pointed out 



I that a series of parts could be distinguished in the mammalian skull, 



I each of which somewhat resemble a vertebra in its general relations, 



, and thus laid a foimdation for a 'vertebral theory of the skull' 



which was farther developed by Owen. Huxley showed that these 



were superficial resemblances, that the three or four vertebrae they 



would recognize were nothing of the sort, and that the skull shows 



no real metamerism except in the occipital region and in the visceral 



arches. 



In its development the skull, like the rest of the skeleton, passes 

 j through two, and in the bony vertebrates, three stages; membranous, 

 cartilaginous and osseous, and in the early stages and in the adult 

 elasmobranchs there is no trace of segmentation or of vertebrae, 

 the Okenian segments only appearing with the development of bone. 

 The skull may be divided into two portions, a cranium, composed 

 of a case for the brain, and sense capsules enclosing 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 (membranocranium) save that it envelops the brain and sense 

 organs, extends into the visceral region, and that it affords the sub- 

 stance in which the second, or cartilaginous, skull is formed. 



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 

 laterally 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 internal ear and joins the para- 

 chordals, thus forming a trough in which the posterior part of the 



