EMBRYOLOGICAL CRITERION OF HOMOLOGY. 1 05 



the ontogenetic ^ phenomena must be sifted apart — an opera- 

 tion that requires more than one critical gramim salts ! On 

 what ground shall this critique be based ? Assuredly not by 

 way of a circulus vitiosus on the ontogeny again ; for if ceno- 

 genetic characters are present in one case, who will guarantee 

 that a second case, used for a comparison with the first, does 

 not likewise appear in a cenogenetic disguise ? If it once be 

 admitted that not everything in development is palingenetic, 

 that not every ontogenetic fact can be accepted, so to speak, 

 at its face value ('als bare Munze '), it follows that nothing 

 in ontogeny is immediately available for the critique of embryo- 

 logical development. This conclusion cannot be escaped. The 

 necessary critique must be drawn from another source." ^ This 

 source is, namely, the facts of comparative anatomy. 



The force of this passage cannot be disputed, and it has led 

 the way in a revolt against the recapitulation theory which has 

 assumed formidable dimensions, especially in England, where 

 within the year one leading morphologist has declared that 

 "von Baer's law falls to the ground," and another has asserted 

 that " the embryological method has failed in so far as the 

 attempt has been made to extend the general proposition {von 

 Baer's law) to particular questions of descent." Most of the 

 protests against the theory have thus far been directed against 

 the view that particular larval or embryonic forms (Trochophore, 

 Nauplius, etc.) can in the totality of their organization be re- 

 garded as representatives of ancestral types, even after allow- 

 ing for a considerable amount of "secondary" (cenogenetic) 

 modification. As far, however, as particular parts or organs 

 are concerned, it is still generally assumed that a safe basis of 

 comparison is to be found in the origin of these parts from 

 particular regions (germ-layers, etc.) of the embryo ; and thus 

 the embryological criterion of homology is still, on the whole, 

 accepted. It is just here, however, that some of the most 

 startling contradictions have recently come to light ; and to 

 certain of these attention will now be called. 



By way of introduction we may first inquire, what is meant 

 by the " embryological origin " of a part or organ. Origin 



1 Cenogenetic obviously intended. ^ Morph. Jahrb., XV, p. 5. 



