50 THE STORY OF FISH LIFE. 



such as the reptiles or birds, or mammals, we 

 should find a skull made up entirely of the 

 bones whose arrival we have just been watch- 

 ing. They have slowly and quietly displaced 

 the cartilage, converting the soft cartilaginous 

 cranium into a strong bony box, and welding 

 with this, in greater and greater perfection, the 

 jaw apparatus ; till at labt the upper jaw be- 

 comes firmly and immovably fixed to the front 

 end of the cranium, and only the lower jaw, 

 now ensheathed in bone, remains movable, 

 Perhaps one of the most interesting features 

 in this transformation of the skull is that which 

 has resulted in the intimate relationship of the 

 plates which originally were only superficial 

 being modifications of the skin with the bony 

 portions of the skull which first appeared within 

 the cartilage forming the capsule of the ear, and 

 the hinder wall of the brain-case. These two 

 kinds of bone, of quite different modes of 

 origin, in all the higher vertebrates form a 

 complex whole, giving no trace of their original 

 very different derivation. 



Those whose work it is to study the history 

 of the development of animals within the egg 

 tell us that much of the history of this 

 development of the skull which we have traced 

 "in time," as we may call it, is repeated in 

 the development of the individual. Thus the 

 bones which we found made their first ap- 

 pearance in cartilage, do so still, and slowly 

 replace it. Those, however, which made their 

 appearance as plates, developed as modifications 

 of the skin, are not preceded by cartilage, but 



