158 THE EMBRYOLOG1CAL CRITERION 



muscle-plates did not extend into the head, and he denied 

 Reichert's assertion (1837) that the cranial basis in mammals 

 showed transverse grooves delimiting three cranial vertebrae 

 (p. 36). The gill-slits, he considered, could not possibly be 

 regarded as marking the limits of head vertebrae. 



In 1858 appeared Huxley's well-known Croonian Lecture, 

 0/t tJic TJicory of the Vertebrate Skull} in which he stated 

 with great clearness and force the case for the embryological 

 method of determining homologies, and criticised with 

 vigour the vertebral theory of the skull. By this time the 

 two rival methods in morphology had become clearly 

 differentiated, and Huxley was able to contrast them, or at 

 least to show how necessary the new embryological method 

 was as a corrective and a supplement to the older anatomical, 

 or, as he calls it, " gradation " method. Applied to the 

 " Theory of the Skull," the gradation method consists in 

 comparing the parts of the skull and vertebral column in 

 adult animals with respect to their form and connections. 

 " Using the other method, the investigator traces back skull 

 and vertebral column to their earliest embryonic states and 

 determines the identity of parts by their developmental 

 relations" (p. 541). This second method is the final and 

 ultimate. " The study of the gradations of structure presented 

 by a series of living beings may have the utmost value in 

 suggesting homologies, but the study of development alone 

 can finally demonstrate them " (p. 541). As an example of 

 the utility and, indeed, the necessity of applying the embryo- 

 logical method Huxley takes the case of the quadrate bone in 

 birds. This bone had been generally regarded by anatomists 

 as the equivalent of the tympanic of mammals, on account of 

 its connection with the tympanum ; but Reichert showed 

 (1837) that the same segment of the first visceral arch 

 developed into the incus in mammals, and into the quadrate 

 in birds, and that therefore the quadrate was homologous 

 with the incus. Similarly, on developmental grounds, the 

 malleus or hammer of mammals is the homologue of the 

 articular of birds, since both arc developed from a portion 



1 Delivered i/tli June 1858. Reprinted in The Scientific Memoirs of 

 T. //. 7/n.r/cy, edited l>y M. Foster and K. Ray Lankcster, vol. i., pp. 



5; v s-r,,,r, (1898). 



