May, 1905.] The Nature of the Reduction Division. 



333 



chromatin as before and from two sources, therefore with some- 

 what different hereditary tendencies. Evidently a fusion or 

 mixing and doubHng of nuclei generation after generation is 

 impossible, especially if the chromatin is organized into definite 

 chromosomes. A reduction division of some kind is the inevitable 

 accompanyment of a conjugation process in the life cycle. This 

 fact being recognized we may proceed to find out in what ways 

 the life cycle may be established. Theoretically a number of 

 possible modes may be developed and the reduction division 

 established at three points in the life cycle. 



a 



b \ 



% 





0^@-(j^-®->(x) 



@ 



Fig. 1. Diagrams of vaiious types of life cycles 



1. Suppose an organism with x chromosomes to give rise to 

 zoospores which conjugate completely; the resulting zygote 

 would have 2x chromosomes. If, however, reduction takes place 

 at the first division or germination of the zygote, the new organ- 

 ism would have the reduced or x number of chromosomes. The 

 double number of chromosomes exists only in the resting stage 

 of the zygote and the result is a simple sexual cycle, the gametes 

 being produced without a reduction division (Fig. la). 



2. If an organism with x chromosomes gives rise to con- 

 jugating zoospores, the zygote will contain 2x chromosomes. 



