THE ENDOSKELETON 



143 



through by the adult ancestors o'f the present-day vertebrates. 

 It is true that the early stages thus indicated do not correspond 

 with the adult condition of any form now living, but of the two 

 types in which we might expect to find a correspondence 

 with this period of the history, A-mphioxus has no head, and, 



!of course, no skull, and the cyclostomes ,with their parasitic 

 habit are too much modified to be reliable ; there is, moreover, 

 an enormous gap between the two and a second, almost as 

 great, between the latter and the selachians, so that it may well 

 be conceded that adult animals representing the stages indi- 

 cated by the embryonic history once existed in the places now 

 left vacant. Nothing could fit better into this ontogenetic 

 history at a later period than the selachian skull, as will be 

 shown further on, thus verifying the record at an important 

 point, and rendering it more probable that the earlier em- 

 bryonic stages, so constant in appearance in all vertebrates, 

 are equally accurate in reproducing the conditions once found 

 in forms now lost to us. To outline the history, then, with the 

 help of embryology, it appears that the ancestral vertebrate, 

 after the acquirement of the prcechordal addition to its head, 

 developed several pairs of external sense organs in the cephalic 

 region, three of which, the nasal sacs, the eyes, and the (inner) 

 ears, have persisted. Of others there are indications in early 

 embryonic life, such as the one placed between the eye and ear 

 and supplied by the seventh nerve, and there are reasons to 

 believe that the original sense organ of the second' pair was 

 not the eye as we have it now, but the lens alone, in the form 

 of a simple capsule; but these matters hardly belong in this 

 place and are suggested merely as indications of the elaborate 

 past history of the head, entirely gone from the world of 

 adult life, but now restored in part by the labors of a gener- 

 ation of embryologists. At this time the notochord, termi- 

 nating at the hypophysis, a downgrowth of the brain just an- 

 terior to the ear capsules, was the only skeletal element in 

 the head, and could have had little value as an organ of sup- 

 port, and none whatever as an organ of protection. This 

 condition of affairs is represented in Fig. 38, A, which, it 



