2l8 



COMPARATIVE ANATOMY 



A further advance towards the skull of higher vertebrates is presented 

 in elasmobranchs, where the fusion and extension of cranial cartilages has 

 produced a brain case which covers the brain except for an anterior and 

 a posterior fontanelle. Among novelties in the skull of elasmobranchs are 

 cartilages homologous with those which give rise to the alisphenoid bones 

 of higher vertebrates. The beginnings of a dermal skeleton appear in 

 this group in the form of placoid scales. The bone-like basal plates of 



OLFACTORY PIT 



5^,-PREFRONTAL 



CHONOROCRANIUM 

 C5TIPPLE0) 



SQUAMOSA It 



POSTFRONTAL 



EXOCCIPITAL 



SUPRAOCCIPITAL 



Fig. 175. — The head of a sturgeon, viewed from above as a translucent object. 

 Membrane bones (scutes) are outlined and the inner cartilaginous cranium stippled. 

 By means of comparative anatomy it is possible to identify certain scutes as homologues 

 of bones in the mammalian skull. (Redrawn after Gegenbaur.) 



these scales are considered as the beginnings of the dermal skeleton and of 

 the membrane bones of the higher vertebrates. 



In ganoid fishes the dermal scales of the head fuse into bony scutes, a 

 number of which (nasal, frontal, parietal, squamosal, etc.) may be traced 

 directly into the membrane bones of man and mammal. The cartilaginous 

 brain case within these dermal plates differs in no essentials from that 

 of elasmobranchs; but a progressive integration and fusion of cartilaginous 

 and dermal constituents explains the two modes of development of bones 

 in the mammalian skull. Intermediate stages in this evolution appear in 

 living and fossil vertebrates. (Fig. 175) 



