616 EMBRYOLOGY. 



place of layers of cartilage, bony tissue upon the already formed 

 cartilage. In the first instance one can speak of an endochondral, 

 in the second instance of a perichoiidral ossification. The cartilaginous 

 primordial skeleton can be crowded out and replaced by a bony one 

 in both ways, remnants of cartilage of greater or less magnitude 

 being preserved in the several classes of Vertebrates. 



The covering bones, on the contrary, arise outside the primordial 

 cranium in the connective tissue enveloping it, either in the skin which 

 covers its surface or in the mucous membrane that lines the head-gut. 

 They are therefore ossifications which do not occur on any other part 

 of the axial skeleton and which are also at first foreign to the skeleton 

 of the head. Consequently in early stages of development, and in 

 many classes of Vertebrates even in the adult animal, they can be 

 dissected off without in any way injuring the primordial cranium. 

 It is otherwise with the primary bones, the removal of which always 

 causes a partial destruction of the cartilaginous skeleton. 



If, as just now stated, the covering bones are at first foreign to the 

 skeleton of the head, there arises the question of their source. To 

 answer this I must go back a little. 



In lower Vertebrates there is developed, besides the internal carti- 

 laginous axial skeleton, an external or dermal skeleton, which serves 

 for the protection of the surface of the body, and is also continued 

 at the mouth for some distance into the cavity of the head-gut, 

 where it may be designated as mucous-membrane skeleton. In the 

 simplest condition it consists, like the scaly armor of the Selachians, 

 of small close-set denticles, the placoid scales, which have arisen from 

 ossifications of dermal and mucous-membrane papillae.- In other 

 groups of the Fishes the dermal armor is composed of larger or 

 smaller bony plates, which bear upon their surfaces numerous 

 denticles or simple spines. They are described according to their 

 form and size as scales, scutes, plates, or dermal bones ; they are 

 explainable in a very simple manner as derivatives from the Sela- 

 chian armor of placoid scales, by the fusion at their bases of larger or 

 smaller groups of denticles, which thus produce larger or smaller 

 skeletal pieces. The larger bony pieces arise principally in the 

 region of the head, and especially at the places where cartilaginous 

 part*; of the cranial capsule or of the visceral arches approach close 

 to the surface. Thus in many Ganoids and Teleosts the brain is 

 found to be enveloped by a double capsule an inner capsule, either 

 purely cartilag'nous or provided with centres of ossification, and a 

 bony armor lying directly upon it. 



