GENERAL OUTLINE 185 



specifically distinct chromosomes is doubled by the union of two 

 germ-cells, a process which if unchecked would quickly lead to an 

 infinite complexity of J:he chromatin or germ-plasm. The end of 

 maturation,; or reduction, is therefore to prevent "the excessive 

 accumulation of different kinds of hereditary tendencies or germ- 

 plasms " 1 through the progressive summation of ancestral chromatins. 



We now come to the vital point of Weismann's hypothesis of 

 reduction, about which all later researches have revolved. Assuming 

 with Roux that the different qualities or "ancestral germ-plasms" 

 are arranged in a linear manner in the spireme-tfrread and in the 

 chromosomes derived from it, he ventured the prediction ('87) that 

 two kinds of mitosis would be found to occur. The first of these 

 is characterized by a longitudinal splitting of the thread, as in ordi- 

 nary cell-division, " by means of which all the ancestral germ-plasms 

 are equally distributed in each of the daughter-nuclei after having 

 been divided into halves." This form of division, which he called 

 " equal division " (Aequationstheilung), was then a known fact. The 

 second form, at that time a purely theoretical postulate, he assumed 

 to be of such a character that each daughter-nucleus should receive 

 only half the number of ancestral germ-plasms possessed by the 

 mother-nucleus. This he termed a "reducing division" (Reduk- 

 tionstheilung), and suggested 2 that this might be effected either by a 

 transverse division of the chromosomes, or by the divergence and 

 separation of entire chromosomes without division. By either method 

 the number of " ids " would be reduced ; and Weismann argued 

 that such reducing divisions must be involved in the formation of 

 the polar bodies, and in the parallel phenomena of spermatogenesis. 



The fulfilment of Weismann's prediction is one of the most inter- 

 esting results of recent cytological research. It has been demon- 

 strated, in a manner which I believe is incontrovertible, that the 

 reducing divisions postulated by Weismann actually occur, though 

 not precisely in the manner conceived by him. Unfortunately, how- 

 ever, this demonstration has been made in only a few specific cases, 

 the complete demonstration, indeed, in but a single group, namely, 

 the copepod Crustacea, while careful studies by the most accom- 

 plished observers have led to an entirely different result in other 

 cases; namely, in Ascaris and the flowering plants. We are in fact 

 confronted by an apparent contradiction of so absolute a character 

 that no middle ground between the conflicting results can at present 

 be discovered. We may best appreciate the nature of this contra- 

 diction by a preliminary consideration of the tetrad groups ; for it 

 is plain that the nature of the maturation-divisions can only be 

 approached through a study of the origin of the tetrads. 



i Essay VI., p. 366. ' I.e., p. 375. 



