412 TRANSACTIONS AND PROCEEDINGS OF [Sess, lyi. 



repeated stimulus of the same kind. It would seem, then, 

 probable that, after a prolonged association in life continued 

 through a series of fissions, the nucleus would respond less 

 readily to the stimuli received from the cytoplast ; conse- 

 quently its directive powers would be diminished ; and, 

 conversely, the protoplasm would do its work more imper- 

 fectly ; the nucleus again would be less nourished, and a 

 vicious circle of deterioration would set up in the cell, ending 

 in senescence and death. To prevent " degeneration and 

 loss of constitutional vigour produced by the over-prolonged 

 association of nucleus and cytoplast, unchanged through a 

 long chain of fissions, the escape lies through a rejuvenescence 

 of the ' firm,' as we may term them." 



This rejuvenescence may be brought about by — 



1. Uest, as in the agamous Monadinea?. 



2. Change of the mode of life by Polymorphism or by 



Heteroecism (as pointed out by Marshall Ward, 

 " On the Sexuality of the Fungi," Quart. Journ. 

 Micr. Sc, 1884). 



3. Nuclear migration, i.e., the transference of a nucleus 



to a portion of cytoplasm with which it has not 

 been associated (in apocytial plants), as through 

 the clamp-connections and anastomoses of the 

 Fungi with septate hypluv. 



4. Plasmodium formation, that is the cytoplastic union 



of cells without nuclear fusion. 



5. Karyogamy, or the fusion of two or more nuclei, as 



well as of their cytoplasts into a uninucleate cell, 

 the zygote. In binary union, the cytoplast of 

 one of the gametes may be practically nil. 

 G Fusion of apocytial gametoids. (Probably only a 

 subdivision of plasmodial formation or karyogamy). 



We see thus that karyogamy, i.e., a union of gametes 

 involving fusion of their nuclei, is equivalent to " the forma- 

 tion of a nucleus new to the cytoplast with which it is 

 associated, a change in the constitution of the ' firm ' and 

 ' staff,' to speak metaphorically," which results in a rejuve- 

 nesced or constitutionally invigorated zygote. 



Ilartog's paper is undoubtedly a great boon to all who 

 study the problems of reproduction, for a clearer and more 



