FOUNDATION OF A THEORY OF HEREDITY. 211 



stances than upon any marked difference in the cytoplasm itself. It 

 is obvious that, in the construction of the embryo, the amount of 

 cell-material must be first of all increased, and that it is only at a 

 later period that the material must be differentiated so as to 

 possess various qualities, according- to the principle of division of 

 labour. Facts of this kind are also opposed to Strasburger's view, 

 that the cause of changes in the nucleoplasm does not lie within 

 this substance itself but within the cell-body. 



I believe I have shown that theoretically hardly any objections 

 can be raised against the view that the nuclear substance of 

 somatic cells may contain unchanged germ-plasm, or that this 

 germ-plasm may be transmitted along- certain lines. It is true 

 that we might imagine a priori that all somatic nuclei contain 

 a small amount of unchanged germ-plasm. In Hydroids such 

 an assumption cannot be made, because only certain cells in a 

 certain succession possess the power of developing into germ-cells ; 

 but it might well be imagined that in some organisms it would 

 be a great advantage if every part possessed the power of growing 

 up into the whole organism and of producing sexual cells under 

 appropriate circumstances. Such cases might exist if it were pos- 

 sible for all somatic nuclei to contain a minute fraction of un- 

 changed germ-plasin. For this reason, Strasburger's other objection 

 against my theory also fails to hold ; viz. that certain plants can 

 be propagated by pieces of rhizomes, roots, or even by means of 

 leaves, and that plants produced in this manner may finally give 

 rise to flowers, fruit and seeds, from which new plants arise. ' It 

 is easy to grow new plants from the leaves of Begonia which 

 have been cut off and merely laid upon moist sand, and yet in the 

 normal course of ontogeny the molecules of germ-plasm would not 

 have been compelled to pass through the leaf; and they ought 

 therefore to be absent from its tissue. Since it is possible to raise 

 from the leaf a plant which produces flower and fruit, it is per- 

 fectly certain that special cells containing the germ substance 

 cannot exist in the plant.' But I think that this fact only proves, 

 that in Begonia and similar plants, all the cells of the leaves 

 or perhaps only certain cells contain a small amount of germ- 

 plasm, and that consequently these plants are specially adapted 

 for propagation by leaves. How is it then that all plants cannot 

 be reproduced in this way ? No one has ever grown a tree from 



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