THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 617 



In the higher Vertebrates the most of the dermal skeleton has com- 

 pletely degenerated, but on the head it is in large part preserved, 

 and furnishes the previously mentioned covering bones, which serve 

 to supplement and complete the internal skeleton. 



An interesting insight into the original method of the development 

 of covering bones can still be acquired in many of the Amphibians 

 (fig. 336). For example, the vomer and the palatinum, which are 

 covering bones, arise in very young Triton 

 larvae by the formation of small denticles 

 (z') in the mucous membrane of the oral 

 cavity, and by the fusion of their bases to 

 form small tooth-bearing plates of bone 

 (z, z). These plates increase in size for 

 a time, owing to the establishment in the Fig. 336. Vomer of an Axoioti 

 neighboring mucous membrane of addi- By ^^^0^^ (z, -) a 

 tional dental spines, which become attached tooth-bearing plate of bone 



,. . . , 1,1 <, ha* 3 arisen in the mucous 



to their margins; afterwards they often membrane. ^ Apices of 



lose the equipment of denticles, which are teebh in P rocess of develop- 



ment, which are subsequently 



destroyed by being resorbed. attached to the margin of the 



It may be said that the original process bon y P late and contrttnite to 



J its growth. 



in the development of covering bones here 



described is abbreviated in most of the Amphibia. For at the places 

 in the mucous membrane which the vomer and the palatinum occupy, 

 the tips of denticles are not even begun ; but in the layer of tissue 

 in which otherwise the bases of the denticles would have been fused, 

 a process of direct ossification takes place. In the same abbreviated 

 way the covering bones arise in all Reptiles, Birds, and Mammals. 



The skulls of many Amphibia (Frog, Axoioti) likewise afford the 

 best explanation of the original relation of the covering bones to the 

 primordial skeleton (fig. 337). The covering bones are found to be 

 loosely superposed upon the primordial cranium, from which they can 

 be easily removed. Thus upon the left side of the accompanying 

 figure the premaxillaria (Pmx), maxillaria (M), vomer (Vo), palati- 

 num (Pal), pterygoid (Pt), and parasphenoid (Ps) have been detached, 

 whereas upon the right side they have been retained. After their 

 detachment there is left the inner head-skeleton proper a capsule 

 still consisting in great part of the original cartilaginous tissue 

 (iV, N l , PP, Qu), into which, however, there are introduced at some 

 places bony pieces : the occipitalia (Olat), petrosa (Pro), sphenoidea 

 [sphenethmoid] (E), etc. 



In the higher Vertebrates, especially in Mammals, the primordial 



