THE ENDOSKELETON 151 



and as the purpose of this procedure is wholly physiological, 

 the object being to insure local strength or gain muscular 

 attachments, the process does not respect origin, but often- 

 times involves both dermal and cartilage bones, and may even 

 include, as well, elements of the visceral skeleton. The re- 

 sults are thus bone-complexes, each of which, in the adult, is 

 a morphological puzzle, to the history of which we have in 

 this stage no clew. Among mammals these consolidations are 

 very extensive, especially among the primates, but even this 

 condition is eclipsed by that in birds, where the fusion reaches 

 its extreme and nearly all of the bones of the cranium proper 

 are fused in the adult into a single piece, so that, in order to 

 properly study the skull, observation must be made upon a 

 newly-hatched fledgeling or even upon an advanced embryo 

 extracted from the egg. 



In the skulls of both amphibians and amniotes, even in the 

 most completely ossified ones, there remain certain portions 

 of the unossified chondrocranium, especially about the nose 

 and internal ears. In mammals there develop from this the 

 cartilages of the external nose which, although often highly 

 specialized, are to be considered in respect to origin the most 

 ancient parts of the skull. 



In giving the account of the earlier stages in the develop- 

 ment of the chondrocranium, nothing was said concerning the 

 subsequent history of the cartilage of the eye-ball, which 

 could not unite in the formation of the skull. In many fish 

 the outer coating of the eye-ball (sclera) remains cartilaginous^ 

 and occasionally becomes very thick and heavy (e. g., sword- 

 fish). There are also traces of cartilage in the sclera of many 

 salamanders. Both cartilage and bone occur in the sclera of 

 certain birds, notably hawks and owls, in the latter of which 

 a series of long, palisade-like ossicles forms an elongated tube, 

 shaped something like the tubes of an opera-glass. Whether 

 these are the direct descendants of the primitive capsule seems 

 very doubtful, and it is more probable that these develop- 

 ments have been called out de novo in response to functional 

 necessity. 



