DEVELOPMENT OF THE SKULL. 355 



In front, at the head end of the embryo, the middle 

 layer (mesodenna) does not split into primitive vertebrae and 

 side-plates, and the original fibrous layer here remains un- 

 divided, forming the so-called " head-plates '" (Fig. 102, Ic, p. 

 337). From these arise the skull the bony covering of the 

 brain as well as the muscles and leather-skin of the head. 

 The skull develops precisely in the same way as the mem- 

 branous vertebral column. The right and left head-plates 

 arch towards each other over the brain-bladder, enclose the 

 anterior extremity of the chorda, and thus eventually form 

 a simple soft, membranous capsule round the brain. This 

 afterwards changes into a cartilaginous primitive skull, 

 similar to that which is retained throughout life by many 

 fishes. It is only much later that the permanent bony 

 skull, with all its different parts, is formed from this cartila- 

 ginous primitive skull. 



In the embryo of Man, as in that of all other Vertebrates, 

 the very remarkable and important structures, which are 

 called the gill-arches and gill-openings, appear, at a very 

 early period, on each side of the head (Plate I. Fig. 1, and 

 Figs. 116, 118, /). . These are among the characteristic and 

 never-failing organs of the Vertebrates, for which reason 

 we mentioned them in considering the typical primitive 

 Vertebrate (Figs. 52, 53, p. 256). On the right and left walls 

 of the intestinal head-cavity, in the anterior portion, first 

 one, and then several pairs of sac-like protuberances are 

 formed, which break through the entire thickness of the 

 side wall of the head. They thus become slits through 

 which there is a free passage from without into the throat- 

 cavity: these are the gill-openings, or throat-openings. 

 Between each pair of gill-openings the wall of the throat- 



