EMBRYOLOGICAL CRITERION OF HOMOLOGY. 115 



define to some extent my own position. I hold, in common 

 with many others, that the ovum is a body composed of a 

 specifically organized substance which may. conveniently be 

 called germ-plasm or idioplasm (for the present purpose we 

 need not inquire whether the idioplasm is contained in the 

 nucleus, in the cytoplasm, or in both). Upon an appropriate 

 stimulus (fertilization, etc.) and under certain definite condi- 

 tions, the idioplasmic organization gradually transforms itself 

 (during the "ontogeny") into another form of organization, 

 namely, that of the multicellular adult body. The egg-organi- 

 zation, no less than that of the adult, must in every species 

 possess a definite and specific character, for the eggs of differ- 

 ent species developing under identical external conditions give 

 rise each to its own appropriate form. Adult homologies must 

 be potentially represented in the idioplasm ; for by no process 

 of casuistry can we escape the fact that every adult character 

 is in some manner involved in the constitution of the idioplasm. 

 But is it, then, necessary to assume that every such character 

 is represented by a definite part or region in the idioplasm — 

 that the germ-plasm is, for instance, a microcosm of biophores, 

 determinants, etc., as Weismann assumes, or that there is a 

 predetermined region of the egg-substance for every adult 

 part .'' According to such a view adult homologies, being repre- 

 sented by homologies between corresponding parts or regions 

 of the germ-plasm, would be really as complete and definite 

 in eggs as in adults. 



I, for one, cannot regard such a view either as logically 

 necessary or as in accordance with known facts. Our igno- 

 rance of the internal constitution of the germ-plasm is so great 

 that we may well be cautious in setting up definite hypotheses 

 regarding its nature; but the facts of regeneration, of hetero- 

 morphosis, of the development of isolated blastomeres are, I 

 believe, fatal to any strictly conceived theory of germinal 

 localization. I believe that facts point on the whole to 

 the conclusion that the idioplasmic organization may be 

 far simpler than that of the adult ; that ontogeny is not 

 merely the transformation of one kind of organization into 

 another, but involves beyond that a steady increase of com- 



