OF THE EARLIER GANOIDS. 329 



into cartilage, and then the cartilage becomes mottled at cer- 

 tain centres with bony points, and gradually ossifies i »to the 

 solid, unyielding substance which forms the internal frame- 

 work of all the mammals and reptiles, and of at letjat two 

 great orders of the fishes. As certainly, however, ay carti- 

 lage makes its appearance before bone in the endo-skeletal parts 

 of animals, does bone make its appearance before the forma- 

 tion of the dermo-skeletal parts. In two months after con- 

 ception, ossification is perceptible in the arms, thighs, and 

 lower jaw of the human foetus ; but not until the close of 

 the fourth month are there the rudiments of nails perceptible 

 on the fingers and toes. In even seventh-month children the 

 nails are very defective, there is no hair, and the teeth are 

 mere semi-cartilaginous points, buried deep in jaws which 

 have acquired the osseous consistency. I am informed by 

 our respected ex-treasurer. Professor Dick, that in the foetus 

 of the horse and cow the endo-skeletal parts are formed, in 

 like manner, before the dermo-skeletal, — the bony framework 

 before the hoofs, hair, and teeth ; in the osseous fishes, too, 

 the foetus has escaped for a considerable time from the egg 

 ere it is furnished with teeth or scales. As a general rule, 

 the development of the dermo-skeletal parts of animals as cer- 

 tainly succeeds the ossification of the internal framework, as 

 that ossification takes place in a previously-existing frame- 

 work of cartilage. But, in direct opposition to this embryo- 

 nic law, we find in the ancient ganoids the development of 

 the dermo-skeletal parts singularly complete. The external 

 skeleton, consisting of scales, plates, spines, and teeth, of solid 

 bone, encrusted with an enamel hard enough to turn the keen 

 edge of a tool, completely covered the external parts of many 

 species of these ancient ganoids, whose internal skeletons 

 were either very partially ossified, as in the Coelacanths, or, 

 as in the Dipterians and most of the Cephalaspians, were not 

 ossified at all. And, looking at this peculiarity, it is quite 



