i6o 



CHORDATE ANATOMY 



constructed cartilaginous skull of cyclostomes represents roughly this 

 stage of evolutionary development. In the cranium of cyclostomes, 

 however, in addition to the parachordal and trabecular cartilages there is 

 an ethmoid plate anterior to the trabeculae, and the beginnings of a tectum 

 covering the brain in the region between the otic capsules. (Fig. 149) 



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^ROSTALIA 



CHONDROCRANIUM 

 (STIPPLED) 



SQUAMOSAb 



OPERCULUM 



OLFACTORY PIT 



i^— PREFRONTAL 



;— POSTFRONTAL 



PAR I ETAL 



EXOCCIPITAL 



■SUPRAOCCIPITAL 



Fig. 150. — 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.) 



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 

 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. See Fig. 150. The 

 cartilaginous brain case within these dermal plates differs in no essentials 



