Chromosomes in Living Organisms . 303 
My developmental studies on the spermatozoids of plants 1 
impressed me with the conviction that the surrender of 
morphological individuality by no means involves, for these 
chromosomes, the loss of physiological individuality. It is 
only on the assumption of the persistence of this physiological 
individuality that it is possible to account for the fact that 
a homogeneous filament in the nucleus of a spermatozoid 
gives rise, in the fertilized ovum, to a predetermined number 
of chromosomes. 
It is established that, in the higher plants, all the nuclear 
divisions which lead up to the formation of the sexual cells 
are normally attended by longitudinal splitting of the chro- 
mosomes, so that the number of the chromosomes remains 
the same throughout. There is no such thing, among plants, 
as nuclear divisions resulting in the reduction by one-half 
of the number of the chromosomes. Such a conception 
involves the assumption that the entire, not longitudinally 
split, chromosomes of the mother-nucleus become separated 
into two groups, each of which goes to form a daughter- 
nucleus 2 . If this be so, then each daughter-nucleus must 
contain only half as many chromosomes as the mother- 
nucleus ; and, in the next generation, each nucleus must 
contain only half as many chromosomes as a daughter-nucleus: 
but nothing of the kind can be observed among plants, a fact 
which has to be taken into account in a consideration of the 
phenomena of heredity. Among animals too, as is shown by 
recent researches, the division with reduction taking place in 
the mother-cells of ovum and spermatozoon is dependent 
upon previous longitudinal splitting of the chromosomes, and 
is therefore referable to normal nuclear division : and even 
were there not sufficient evidence to prove this, the facts are 
so clearly ascertained with regard to plants, in which the 
phenomena of heredity and variation are essentially the same 
1 Schwarmsporen, Gameten, &c., p. 145. 
2 Weismann, Ueb. die Zahl der Richtungskorper und ihre Bedeutung fur die 
Vererbung, p. 79, 1894. 
