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. 266), 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 early period of 

 development; these one might be tempted to refer to five 



Fig. 266. — Primitive skull of human 

 embryo of four weeks ; vertical section, 

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

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

 cavity, in which lie the live 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. The 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 



