l62 



CHORDATE ANATOMY 



from that of elasmobranchs ; but a progressive integration and fusion of 

 cartilaginous and dermal constituents explains the two modes of develop- 

 ment of bones in the mammalian skull. Intermediate stages in this evolu- 

 tion appear in living and fossil vertebrates. 



Among the noteworthy changes in the skull during its evolution from 

 fishes to man is the considerable reduction in the number of bony elements. 



Professor W. K. Gregory points out 

 that the primitive fishes have as 

 many as i8o skull bones while 

 higher fishes have only about loo. 

 In amphibians they number from 95 

 to 50. The skulls of earlier fossil 

 reptiles had 80 bones while those of 

 the highly specialized modern 

 snakes have only 50. There were 

 70 bones in the skulls of tertiary 

 reptiles from which mammals 

 evolved; but mammals in general 

 have half that number and the 

 skulls of primates do not have more 

 than thirty bones. Pre- and post- 

 frontals are present in the reptilian 

 skull but are wanting in mammals. 

 Fig. is3.-Skun of a stegocephaian c^j-iously the peccaries, like the 



(Capitosaurus). eo, exoccipital; ep, tabu- ■' . 



lare; /, frontal; ju, zygomatic (jugal); la, birds, have the entire skull fuScd 



lacrimal ;,«x. maxilla ;«a, nasal ;o, orbit ;^a, ^^ ^ continUUm and Only the 



parietal; pmx, premaxiUa; por, postorbital; _ -' 



prf, prefrontal; ptf, postf rental; qj, quad- loWCr jaW a Separate bone. 



ratojugal; 50. dermoccipital; 59, squamosal; ^j^j^ reduction is especially 



,s/, supratemporal. (From Kmgsleys _ _ ^ •' 



"Comparative Anatomy of Vertebrates," striking in the evolution of the 



after Zittel.) dermal skeleton. As the dermal 



scales sink into the deeper layers of the skin, they unite with the bony ele- 

 ments which are preformed in cartilage so that there frequently results a 

 complex bone that has both cartilaginous and membranous elements. 



The tendency of the primitive cartilaginous brain case to become bony 

 begins in the ganoids, advances in amphibians and reptiles and is nearly 

 completed in birds and mammals. Although the advantages of a bony 

 skeleton for land animals is obvious, it is difficult to explain by any hypo- 

 thesis of Lamarck, Darwin or DeVries the substitution of bone for cartilage 

 in aquatic animals. Elasmobranchs such as the dogfishes seem as success- 

 ful in the struggle for existence as are the teleosts. The former have a 

 cartilaginous and the latter a bony skeleton. Cartilage is not a prere- 

 quisite condition for the appearance of bone since membrane bones develop 

 directly from mesenchyma without an intermediate cartilaginous stage. 



