126 AMPHIMIXIS OR ESSENTIAL MEANING OF [XH. 



retained the conception in its original form, as it is indispensable 

 for the understanding of the ' reducing division.' When I main- 

 tained that the units of the germ-plasm are indivisible, I did not 

 refer to mechanical divisibility, but to that division which a 

 unit cannot undergo without losing its original character. If we 

 divide a dog into two parts, neither part is a dog ; and so, accord- 

 ing to my views, half an ancestral plasm is not an ancestral 

 plasm, is not an hereditary unit capable of calling forth a com- 

 plete individual ; or, to express this with reference to its minute 

 structure, a half would no longer contain all the predispositions 

 necessary for the whole individual. The number of these units 

 would be doubled as the result of each fresh fertilization if the 

 preliminary halving did not occur. Hence the necessity for 

 such halving, which I have attem.pted to render clear by the 

 foregoing train of thought. 



Taking my stand upon this, I argued that a ' reducing division' 

 of the nuclear material takes place before fertilization in both 

 germ-cells, — that is a division contrary to the ordinarj'^ method, in 

 that it does not divide the collective ancestral plasms in two equal 

 and similar groups between the daughter-nuclei, as in ' equal 

 divisions^,' but halves their number so that one daughter 

 nucleus receives one set and the other another set of ancestral 

 plasms. In the ovum I recognised the necessary ' reducing 

 division ' in the formation of the second polar body, for it had 

 then been shown by the careful observations of van Beneden and 

 Carnoy upon Ascaris megalocephala that two out of the four 

 nuclear rods pass into the second polar body while the other 

 two compose the nucleus of the ovum. 



The idea of a ' reducing division,' as I then conceived it, seems 

 to have met with but little acceptance among the German 

 biologists. Except Platner and recently O. Hertwig and 

 Henking, I know of no one who has accepted it. The first- 

 named employed the expression, but without indicating 

 whether he used it in my sense. This cannot be taken for 

 granted, as the simple halving of the chromatin mass may 

 be so designated. All that we can see is a reduction in 

 mass, . and the discoveries of Platner and Hertwig do not 

 directly teach us more than that in the division of the mother- 



^ For a further account of these methods of division see Vol. I. pp. 

 369-377- 



