XII.] CONJUGATION AND SEXUAL REPRODUCTION. 1 15 



halves in order to form the nuclei of the daughter-cells. But 

 these halves are not always qualitatively alike ; they are only 

 so when they are to give rise to similar cells : when the cells 

 which arise by division have a different significance in develop- 

 ment, their idioplasm also differs in quality. The germ-plasm 

 of the ovum is thus continually undergoing change during ontogeny^ 

 inasmuch as the developmental tendencies are being split up, and 

 become m,ore and more distributed among the members of successive 

 cell generations, until finally each kind of cell in the body contains 

 only that developmental tendency which corresponds with its 

 specific histological character. Each specific cell is thus domi- 

 nated by a specific idioplasm. 



As soon as I had arrived at this conclusion, it was easy and 

 indeed inevitable to refer the differences between spermatozoon 

 and egg-cell to a specific idioplasm which had stamped its 

 peculiarities upon each cell. But since both male and female 

 germ-cells contain the substance which fuses during fertilization 

 to form the segmentation nucleus, and therefore germ-plasm, I 

 concluded that a part of this true germ-plasm which forms the 

 nuclear substance, splits ofi' at the first ontogenetic stage, as 

 specific sperm or egg idioplasm, which controls the germ-cell 

 during its growth, and confers upon it a specific histological 

 character. I sought for the meaning of the cell-division which 

 results in the separation of the polar bodies, in the suggestion 

 that by this means the spermogenetic or ovogenetic idioplasm, 

 rendered superfluous after the attainment of the specific form, 

 was removed from the germ-cell, while the germ-plasm, grown 

 in the mean time to a larger mass, remained behind in the cell. 

 I therefore recognised in the cutting-ofif of the polar bodies the 

 removal of histogenetic idioplasm from the germ-cells. 



While I was busy working out these interpretations, I dis- 

 covered new facts which caused a modification of this view and 

 led to the conclusion, which up to the present time appears to 

 be sound, that the formation of polar bodies is a process for the 

 reduction of the hereditary substance. 



The fact which led to this conclusion was the law of the 

 number of polar bodies,— the discovery that all animal eggs which 

 require fertilization expel two polar bodies, one after the other, 

 while all true parthenogenetic eggs give rise to one only. Now 

 the ovogenetic idioplasm cannot, at the most, occupy more 



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