100 TIIK EMBRYOLOGICAL CRITERION 



run counter to the homologies indicated by the study of 

 adult structure. " Thus, for instance, although the lower 

 jaw in position, function, form and shape, appears to be 

 the same bone throughout, yet it must be admitted that it 

 shows a difference in the different classes. In Mammals and 

 Man it is an entirely secondary bone (an extremity 

 according to Reichert), in Birds, Amphibia and Fishes only 

 partially so, for its articular belongs to Meckcl's cartilage 

 and is accordingly analogous to a rib ; indeed, in the 

 Plagiostomes, etc., the whole lower jaw along with the 

 articular is a persistent Meckel's cartilage" (p. 290, 1850). 



So, too, the supraoccipital in man cannot be fully homo- 

 logised with the supraoccipital of many mammals, for its upper 

 half arises at first in isolation as a secondary bone (p. 290). 



Reichert objected to the distinction drawn by Kolliker, 

 and denied that there was either a histological or a 

 morphological difference between membrane and cartilage 

 bones. It was shown a few years later by H. M tiller 1 that 

 there was in truth no essential difference in histological 

 development between the two categories of bone, that the 

 cartilage cells were replaced by bone cells identical with 

 those taking part in the formation of membrane bones. 

 The morphological distinction continued however to be 

 recognised, particularly by the embryologists. Rathke in 

 his volume of 1861 ' 2 classified the bones of the skull 

 according to their origin from the primordial cranium or 

 from the overlying fibrous layer, distinguishing as membrane 

 bones, the parietals, frontals, nasals, lachrymals, maxillaries 

 and prcmaxillaries, jugals, tympanic, parts of the" temporal," 

 vomer, part of the supraoccipitals in some mammals, and the 

 mandible (with the exception of the articular in such as 

 have a quadrate bone). Huxley was also inclined in 1864 :! 

 to recognise the distinction, but he writes with some 

 reserve: "Is there a clear line of demarcation between 

 membrane bones and cartilage bones? Are certain bones 

 always developed primarily from cartilage, while certain 

 others as constantly originate in membrane? And further, 



I /.cits. f. MSS Zi>o/., ix., 1858. 



-' Kntiv. d. \\'irl>clthic>\\ pp. 139-40, 1861. 



II Lectures on the Elements of Comparative Anatomy. 



