THE SKELETON OF THE VERTEBRATE HEAD. 95 



discussion of this question has afrorded, I am inclined to think 

 that a histological l)ias has influenced both the views which 

 have been taken of it. "NVhy certain bones originate in a 

 fibrous matrix, why others originate in cartilage which has 

 been previously fonned in the same matrix, are questions of 

 undoubted importance, but which at the same time cannot 

 legitimately be i)ut in opposition to the unity of the fully-de- 

 veloped sclerome. 



Of Hie Cartilaginous and Bony Elements, ami of th^ general 

 MorpJiological Coiutitution of the Sclerotome. — A sclerotome is, 

 fundamentally, a segment of the fibrous sclerome, and the 

 series of fibrous sclerotomes is indicated by the actinal laminje, 

 each of which, for reasons to be afterwards stated, ought pro- 

 bably to be considered as potentially double — that is, as con- 

 sisting of two layers, one belonging to the sclerotome behind, 

 the other to the sclerotome in front. 



The fully developed htemapod sclerotome is therefore a 

 fibrous structure, in which all the cartilaginous and osseous 

 parts are formed and imbedded. With regard to these carti- 

 laginous or osseous elements, I shall at present only direct 

 attention to certain points which bear on the constitution of 

 the sclerotomes of the head. In doing so, I must bear testi- 

 mony to the general applicability and convenience of the 

 terms employed by Professor Owen to designate the elements 

 of his typical vertebra, venturing to suggest modifications in 

 their application only where I am compelled to difter from 

 him in regard to the relations of the elements themselves. 



The term "centrum" is highly useful as a designation for 

 the caiiUaginous or osseous mass formed around the " corda 

 dorsalis," whatever the constitution of that mass may be. 



The neurapophyses or hard parts developed in the lateral 

 neural lamina) are " typically " two at least on each side. Not 

 only are there two on each siile in the tnink sclerotomes of 

 certain cartilaginous and probably osseous fishes ; but there 



