FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS 343 



in this, namely, that they are mutually attractive ; they find each 

 other and unite, and then go on to develop, which each was previously 

 unable to do by itself. Widely as the sperm-cell and egg-cell differ 

 in size, constitution, and behaviour, in regard to essential character 

 they are alike ; they bear the relation— as I expressed it twenty years 

 ago — of I : I ; that is, they both contain an equal quantity of essentially 

 siniilai' herecUtajry substance, and the quality of this substance is 

 only individually variable. We should, therefore, speak not of a 

 ' male ' and ' female,' but of a ' paternal ' and a ' maternal ' nucleus. 



All the more recent experiments on ' merogony,' that is, on the 

 development of fragments of the ovum, confirm this view. Thus 

 Boveri had already observed that even small pieces of sea-urchin ova 

 which did not contain the nucleus of the ovum developed, after the 

 spermatozoon had entered them, into small but otherwise normal 

 larvaj of the species. More recently Hans Winkler proved the same 

 thing for the ova of plants, by dividing the ovum of a marine alga 

 (Cystosira) into two pieces, then fertilizing these with water con- 

 taining sperms, with the result that he got from both pieces, the 

 nucleated and the non-nucleated, an embryo of normal appearance. 

 In the latter it could only have been a ' paternal ' nucleus which 

 directed the development. 



To sum up. Our investigation into the meaning of amphimixis 

 has led us to the conclusion that it consists in the union of two equal 

 complements of hereditary substance, contributed by two different 

 individuals, into one unified nucleus, and that the sole immediate 

 result of this is the combination of the hereditary tendencies of two 

 individucds in one. Among multicellular organisms this one indi- 

 vidual of dual origin always implies the beginning of a new life, since 

 amphimixis is indissolubly associated with reproduction, and even 

 among unicellular organisms it can hardly be disputed that the two 

 Infusorians which separate after conjugation are no longer the same 

 as they were before. After amphimixis they must contain a dif- 

 ferent combination of hereditary substance from what they had 

 before, and this must reproduce the parts of the animal in a some- 

 what modified form. This is theoretically beyond douljt, although it 

 can scarcely be established by observation. 



We thus know now what ' fertilization ' is. Through the labours 

 of the last decade the veil has been torn from a mystery of nature 

 which for thousands of years confronted humanity as unapproachable ; 

 a riddle has been solved for the solution of which a few centuries 

 ago men did not even dare to hope. Not a few have taken part in 

 these labours ; some I have already named, but it is impossible that 



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