IV.] FOUNDATION OF A THEORY OF HEREDITY. 21 



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ance of the cells of the ectoderm and endoderm in sponges and 

 Coelenterata) perhaps depend more largely upon a different 

 admixture of nutritive substances 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 objec- 

 tions 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 con- 

 tain 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 organ- 

 isms 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 possible for all somatic nuclei to contain 

 a minute fraction of unchanged germ-plasm. 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 oi 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 perfectly 

 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 per- 

 haps only certain cells contain a small amount of germ-plasm, 



