160 THE EMBHYOLOGICAL CRITERION 



features to the ossified skull of other Vertebrates; the 



relations of the ear to the vagus and trigcminal nerves are, 



for instance, the same in both ; the main regions of the 



cartilaginous skull can be homologised with definite bones or 



groups of bones in the bony skull ; but discrepancies occur. 



It is again to development that we must turn to discover the 



true relationship of the cartilaginous to the ossified skull. 



" The study of the development of the ossified vertebrate 



skull . . . satisfactorily proves that the adult crania of the 



lower Vertebrata are but special developments 1 of conditions 



through which the embryonic crania of the highest members 



of the sub-kingdom pass" (p. 5/3). It is with the embryonic 



cranium of higher Vertebrates that the adult skull of the 



lower fishes must be compared, and the comparison will 



show a substantial though not a complete agreement 



between them. Thus, speaking of the development of the 



frog's skull, Huxley writes : " If, bearing in mind the changes 



which are undergone by the palatosuspensorial apparatus, 



. . . we now compare the stages of development of the frog's 



skull with the persistent conditions of the skull in the 



Ainpliioxns, the lamprey, and the shark, we shall discover the 



model and type of the latter in the former. The skull of the 



Ajiip/iio.vus presents a modification of that plan which is 



exhibited by the frog's skull when its walls arc still 



membranous and the notochord is not yet embedded in 



cartilage. The skull of the lamprey is readily reducible to 



the same plan of structure as that which is exhibited by 



the tadpole \vhen its gills are still external and its blood 



colourless. And finally, the skull of the shark is at once 



intelligible when we have studied the cranium in further 



advanced larvae, or its cartilaginous basis in the adult frog" 



(P- 577)- Development, therefore, proves what comparative 



anatomy could only foreshadow the unity of plan of all 



vertebrate skulls, ossified and unossified alike. "We have 



thus attained to a theory or general expression of the laws of 



structure of the skull. All vertebrate skulls arc originally 



alike; in all (save .li/ip/i io.v/is ?} the base of the primitive 



cranium undergoes the mcsoccphalic flexure, behind which 



the notochord terminates, while immediately in front of it 



1 Cf. Rcichert, st//>m, p. 149. 



