SIXTH LECTURE. 



THE EMBRYOLOGICAL CRITERION OF 

 HOMOLOGY. 



EDMUND B. WILSON. 



THE word homology is at present generally employed to 

 denote two widely different kinds of morphological likeness. 

 One of these is the similarity between corresponding parts of 

 different individuals (hence equivalent to the " special homol- 

 ogy" of Owen), and this we regard as a result of the descent 

 of those individuals from a common ancestor; such a resem- 

 blance may, therefore, be designated as a genetic homology. 

 The other is the similarity between corresponding or repeated 

 parts in the same individual (including the " serial homology" 

 of Owen, or homodynamy of Bronn, and the " homonomy " of 

 Haeckel); this may be called meristic homology, adopting the 

 convenient word recently introduced by Bateson. We shall 

 here deal only with genetic homologies (which form the ground- 

 work of all systems of classification), and especially shall 

 attempt to consider the criteria employed in their determi- 

 nation as they appear in the light of recent advances in embry- 

 ology. 



I. 



As originally defined by Owen, genetic (i.e., " special ") 

 homology was based entirely upon anatomical data. It was 

 a morphological correspondence in the " relative position and 

 connexions " of parts without regard to their mode of develop- 

 ment ; indeed Owen expressly repudiated that " loose use" of 

 the word to denote similarity of development of which he 

 accused Geoffrey St. Hilaire and the German anatomists. 



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