m] OTHER DEFINITIONS 67 



One of the most widespread definitions considers 

 the individual as "the total product of a single 

 impregnated ovum " (8 a, p. 59), that is to say as the 

 sum of the forms which appear between one sexual 

 act and the next. This would make all the polyps 

 in a colony of hydroids, all the separate polyps 

 budded off by a fresh-water hydra, all the summer 

 generations of the aphis, together constitute but 

 a single individual. Of recent years it has not found 

 so much favour, but Calkins (2) has urged that it 

 should apply to protozoa, declaring that all the 

 separate cells arising by continued division from 

 a single parent between one sexual act (conjugation) 

 and the next, should be considered as one individual, 

 no less than the cells of a metazoan like man, 

 which too arise by continued division from a single 

 parent, the ovum, and remain connected to form his 

 body. 



Of the various facts which make the hypothesis 

 untenable, the chief are concerned with the artificial 

 or accidental production of two or more co-existent 

 organisms from a single ovum. 



In most animals each single fertilized egg gives 

 rise to a single embryo and this to a single adult 

 organism: but in some, where this is the normal 

 rule, more than one embryo may be accidentally or 

 artificially formed from one egg, and in others this 

 multiplicity is the usual course of events, even 



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