218 



CYTOLOGY 



Eduard Strasburger 



The Periodic Reduction of the Number of 

 the Chromosomes in the Life-history of 

 Living Organisms 



Reprinted with the permission of the publisher 

 from Annals of Botany 8:281-316, 1894. 



The simplest organisms with which 

 we are acquainted reproduce them- 

 selves in only an asexual manner. It 

 would appear that it is only in the 

 lowest organisms that the absence of 

 sexual differentiation is possible, and 

 that this differentiation necessarily ac- 

 companies a certain definite degree of 

 organization: it is, in fact, as if this 

 differentiation must manifest itself at 

 a certain stage of phylogcnetic evolu- 

 tion in virtue of certain properties pos- 

 sessed by organized matter as such. It 

 is true that many highly organized 

 plants are asexual, but comparative in- 

 vestigation proves that this is due to 

 a gradual loss of sexual differentiation, 

 as in the great group of the fungi, and 

 doubtless also in the apogamous ferns. 



It appears that the sexual act has 

 always given a powerful impulse to 

 phylogcnetic evolution; and that, on 

 the other hand, all advance in develop- 



ment was in abeyance so long as 

 sexual differentiation had not been ob- 

 tained. From the phylogcnetic stand- 

 point, we must assume that all sexually 

 differentiated organisms are descended 

 from asexual organisms. 



Our insight into the nature of the 

 process of fertilization was very ma- 

 terially promoted by the discovery, 

 made by Edouard van Beneden, that 

 the number of the chromosomes is the 

 same in both the conjugating nuclei. 

 Further investigations established the 

 fact, for both animals and plants, that 

 a reduction to one-half of the number 

 of the chromosomes in the generative 

 nuclei precedes the sexual act, and that, 

 in consequence of the coalescence of 

 the male and female nuclei, the nu- 

 cleus of the fertilized ovum possesses 

 the number of chromosomes character- 

 istic of a vegetative cell. 



Guignard and I established the 



