Chromosomes in Living Organisms. 305 
endowed with all the characteristics of the species ; but their 
activity is stimulated in a definite direction by the prevalent 
conditions. Were this not the case, it would be impossible 
for the renewed development of organs to take place, as it 
does, from any part of the plant-body, organs which mani- 
fest all the characteristics of the species ; nor would it be 
possible to stimulate certain activities by artificial inter- 
ference, and to induce this or that manifestation of hereditary 
capabilities. It is in this way that I account for the influence 
of those external conditions, for instance, which determine 
sexual or asexual reproduction in the Algae ; as also for 
the influence of certain substances, formed by the organism 
itself, which induce the formation of a flower at the growing- 
point. 
I have rejected the view of the hereditarily unequal division 
of nuclei on the ground that it is contrary to the facts 
ascertained by direct observation, and I am equally unable 
to admit that theories of heredity are justified in recon- 
structing the nucleus with the object of finding in it all the 
structures which are necessary to them : the only legitimate 
point of departure is afforded by the actually observed facts 
of nuclear structure. I consider Weismann’s conception of 
the id 1 , as an element in the nucleus which is charged with 
all the hereditary characteristics of the species, to be felici- 
tous, because it appears to me that it can be supported by 
direct observation. I regard as ids the discoid segments of 
the chromosomes, which are all exactly similar in form and 
structure, and are serially arranged with such remarkable 
regularity in the chromosomes of nuclei about to divide. 
Whilst in the resting-stage of the nucleus the substance 
of each id is distributed, for nutritive purposes, over the 
elongated nuclear filament, in the prophase the substance 
segregates to constitute and form a segment of the series. In 
the id there are represented not only the small chromatin- 
1 Das Keimplasma, eine Theorie der Vererbung, p. 84 : p. 60 in the English 
edition, 1893. 
