Vertebroid Homologies of the Cranium in Vertebralia. 59 



the humeral zone, which, in insects, carries on the haemal 

 upturned aspect, the first pair of wings, appendages of the 

 respiratory organs, whether lungs, branchias, tracheae, &c. &c 



The metathoracic ring supports the hind legs, and the 

 second pair of wings on the hseinal aspect, representing the 

 ventral fins in fishes. 



In Crustacea the number of legs from the mesothoracic 

 and metathoracic rings are doubled ; the prevertebral cara- 

 pace homologous with the mammal palate completely covers 

 the centres of the nervous, digestive, and vascular systems ; 

 these rings are incomplete. 



A portion of Professor Huxley's " Lectures on the Verte- 

 brate Skeleton" has appeared in " The Lancet," illustrated 

 by diagrams in wood. These have been little consulted, 

 from being inconsistent with the vertebral scope of this paper, 

 based on the unity of organisation and the vertebroid homo- 

 logies of the animal kingdom. " Who can decide when 

 doctors disagree ?" 



On Development as the basis of Homology. 



Agassiz, Goodsir, Huxley, and others, maintain that the 

 study of the progressive development of the various lamina) 

 composing the skelon is the only basis on which the deter- 

 mination of accurate homology can surely rest ; and Cuvier 

 long ago propounded the same dogma when enumerating the 

 bones in each species of the mammalia. " We must descend 

 to the primitive osseous centres as they exist in the foetus." 

 Owen showed the inapplicability of this rule, as the human 

 brachium should be counted three bones instead of one, and 

 in like manner four would be enumerated instead of the 

 femur. The Cuvierian rule fails by not distinguishing be- 

 tween the ossific points which permanently complete the 

 bone and those which only typify parts of bones which are 

 in the foetal condition separate, in order to facilitate ossifi- 

 cation of individual bone, as the cases above referred to. The 

 study of development having comparatively but a restricted 

 sphere among anatomists and naturalists, little advantage 

 can safely be taken of it, as in the present state of our 

 knowledge it seems to have led as much to confusion as to 



