144 



HISTORY OF THE HUMAN BODY 



must be noted, represents the head as seen, not from above, 

 as it is more usually drawn, but from below, a view that en- 

 ables one to see the notochord and its termination behind 

 the hypophysis. To this condition there are added, but no 

 one yet knows how or from what source, two pairs of later- 

 ally placed cartilages (Fig. 38, B), the one alongside the noto- 



FIG. 38. Diagrams showing the development of the primordial skull. 

 Since this organ develops primarily beneath the brain as a support the 

 figures represent the ventral aspect. 



(A) Early stage, before the appearance of cartilage. The notochord is seen lying 

 along the nerve cord as far forward as the hypophysis. The three sense-organs, 

 nose, eye, and ear, have already appeared. (B) This stage shows the trabeculae [t], 

 the parachordals [p], and the capsules around the sense-organs. (C) In this the 

 trabeculae, the parachordals, and the nasal and otic capsules have fused into a single 

 mass, the primordial skull, or chondrocranium. The anterior end of the notochord 

 is imbedded in this. The cartilaginous capsule of the eye remains free to allow 

 the necessary movements of the eyeball. 



chord and the other anterior to it, the parachordal and prce- 

 chordal elements respectively. The former are rather flat, of 

 an elongated crescentic shape, filling in the space between tl 

 notochord and the ear capsules; the latter are elongated and 

 rod-like or beam-like, hence often termed the trcibeculce (di- 

 minutive of trdbs, trabis, a beam), and lie a little beneath the 

 eyes and nearer the median line. 



At the same time the three persisting pairs of sense organs 



