THE SKELETAL SYSTEM 



219 



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 180 skull bones while higher fishes have only about 100. In amphibia 

 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 

 pidSS' Ahe 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 postfrontals are 

 present in the reptilian skull but 

 are wanting in mammals. Curio- 

 usly the peccaries, like the birds, 

 have the entire skull fused to a 

 continuum and only the lower jaw 

 a separate bone. 



Thi\ rpdiirl-inn k f-<:;npriallv ^'^- 1 76.— Skull of a stegocephalan 



iniS ^reauction is especially (^Capitosaurus) . eo, exoccipital; «/., tabu- 



Striking in the evolution of the lare; /, frontal; ju, zygomatic (jugal); la, 



Hprmnl <;kplptnn Aq thp Hprmal lacrimal; w*, maxilla; «a., nasal; o, orbit; ^a. 



dermal skeleton. As tlie dermal parietal; pmx, premaxilla; por, postorbital; 



scales sink into the deeper layers prf, prefrontal; ptf, postfrontal; qj, quad- 



of the skin, they unite with the T^^':Sn^ tilT^T's 



bony elements which are preformed "Comparative Anatomy of Vertebrates," 



in cartilage so that there frequently ^ ^^ 1 e .; 



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 amphibia 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 successful in the struggle for existence as are the teleosts. The former 

 have a cartilaginous and the latter a bony skeleton. Cartilage is not a 

 prerequisite condition for the appearance of bone since membrane bones 

 develop directly from mesenchyma without an intermediate cartilaginous 

 stage. The difficulty of explanation of the emergence of bony skeletons 

 from cartilaginous beginnings is greatly increased by the complexity of 

 the processes by which in ontogenesis, and therefore presumably in 



