13G 



LECTURE VIII. 



ON THE STRUCTUEE OF THE SKULL. 



THE DEVELOPMENT OF THE HUMAN SKULL. 



As might be expected from the nature of the case, it lias not 

 yet been possible to obtain a series of human embryos, in every 

 stage of development, sufficiently large to enable embryologists 

 to work out all the details of the formation of the human skull. 

 But all higher vertebrate embryos so nearly follow one and the 

 same type of early developmental modification, that we may 

 reason, with perfect confidence, from the analogy of the lower 

 Vertebrates to man, and fill up the blanks of our observations 

 of human embryos by investigations of the chick, the dog, the 

 rabbit, or the pig. 



In the chick,* the first indication of the body of the embryo 

 is an elongated, elevated area of the blastoderm, the axis of 

 which is traversed by a linear groove. The one end of the 

 elongated area is wider and more distinctly raised up from the 

 rest of the blastoderm, than the other : it is the cephalic end 

 (Fig. 31, A, a), and the linear groove stops short of the rounded 

 extremity of this part of the elevated area. A peculiar cellular 

 cylinder, tapering oft' at each end, the notochord, is soon dis- 

 cerned occupying the bottom of this groove, beneath the outer, 

 serous, or neuro-epidermic layer of the germ. 



A laminar outgrowth of the convex summits of the ridges 

 Avhich bound the primitive groove now takes place, in that part 

 of the embryo, which will eventually become the middle region 



* See Lecture IV., pp. G4— 6G. 



