EVOLUTION OF THE SKULL. 



297 



their primordial separate condition now appears effaced by 

 the action of the "law of abridged heredity," and is no 

 longer demonstrable in the Ontogeny. 



In the human primitive skull (Fig. 2GG), and in that of 

 all higher Vertebrates, which has been modified, phyloge- 

 netically, from the primitive skull of the Selachii, five con- 

 secutive divisions are visible at a certain earl}^ period of 

 development; these one might be tempted to refer io five 



Fig. 266. — Primitive skull of human 

 embryo of four weeks ; vertical section, 

 the left half seen from the inside : v, z, 

 m, h, n, the five grooves in the skull 

 cavity, in which lie the five brain-bladders 

 (fore-brain, twixt-brain, mid-brain, hind- 

 brain, after-brain); 0, pear-shaped pri- 

 mary ear- vesicle ; a, eye ; no, optic nerve ; 

 p, canal of the hypophysist; t, central 

 part of the cranial basis. (After Koelliker.) 



original primitive vertebrae ; they are, however, merely the 

 result of adaptation to the five primitive brain- bladders, 

 and, like the latter, they rather correspond to a larger 

 number of metamera. Tho fact that the primitive verte- 

 brate skull is a much modified and profoundly transformed 

 organ, and by no means a primitive structure, is also evi- 

 dent in the circumstance that its rudiment, originally a soft 

 membrane, commonly assumes the cartilaginous state only 

 at its base and on the sides, while it remains membranous 

 at the skull-roof Here the bones of the later bony skull 

 develop in the soft membranous rudiment as an external 

 bony roof, without a previous intermediate cartilaginous 

 state, as in the base of the skull. Thus a great part of the 

 skull-bones originally developed as roof-bones from the 



