REPRODUCTIVE ELEMENTS IN APUS AND BRANCHIPUS. 201 



Although both the eggs and the spermatozoa are cells, and 

 similar cells, the final karyokineses which produced them 

 are different from the preceding divisions in the cells of the 

 genital epithelium, whether male or female. As is now well 

 known, this change in the divisional phenomena appears in the 

 extrusion of the polar bodies in the egg, and the " Reductions 

 Theilung"in the spermatozoa. The existence of these phe- 

 nomena constitutes the empirical though important ground 

 for several well-known theories concerning the physiological 

 value of the ''Reductions Theilung" as a preparatory balancing 

 of the "hereditary substance" in two cells, whose fusion forms 

 the starting-point of a succeeding generation. 



Nearly all the readings of this riddle actually offered, have 

 sprung from one of two sources: either they have come back 

 upon us as the new-clad ghosts of Balfour's famous interpreta- 

 tion of the polar bodies; or have accepted as sufficient explana- 

 tion of the facts, Weismann's theoretical conceptions of the 

 necessity of a reduction in the quantity or the quality, or both, 

 of the hereditary substance (chromatin). 



The continuous processes of assimilation and growth in the 

 resting cells of any tissue, although beyond our actual scrutiny, 

 offer nothing antagonistic to the generally adopted notion, that 

 they are the result of the passive influence of an immensely 

 complicated structure, operating under certain rather limited 

 conditions. But the final dissolution of these conditions, when 

 assimilation and growth in the individual can presumably go 

 no further,^ and their re-establishment under more favorable 

 circumstances, is quite another matter. 



In this procedure, the essential material constituents of the 

 cell are accurately halved and separated, and we may presume 

 that the complicated mechanical basis of the resting celPs 

 activity, or at any rate the power to return to it at some future 

 time, goes with them. What are the means by which this 

 material distribution is effected ? According to more recent 



* Herbert Spencer suggests that cell division becomes a necessity in virtue 

 of the continual decrease of the absorptive area proportionally to the growth 

 of the cell. 



