FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS 341 



is not the smallest ground for the assumption that the cell-body as 

 well as the nucleus contains the hereditary qualities, since we find in 

 general that functions are distributed among definite substances and 

 parts of the whole organism, and it is just on this division of labour 

 that the whole differentiation of the body depends. And why should 

 this principle not have been employed just here where the most 

 important of all functions is concerned ? Why should all living 

 substance be hereditary substance ? Although Nageli thought of his 

 ' idioplasm ' otherwise than we now think of hereditary substance, 

 although he wrongly imagined it in the form of strands running 

 a parallel course through the cell-substance and forming a connected 

 reticulum throughout the whole body, he recognized at least so much 

 quite correctly, that there are two great categories of living sub- 

 stance — hereditary substance or idioplasm, and ' nutritive substance ' 

 or trophoplasm, and that the former is much smaller in mass than the 

 latter. We now add to this, that the idioplasm must be sought for 

 in the cell-nucleus, and indeed in the chromatin granules of the 

 nuclear network and of the chromosomes. 



But incontrovertible proof of the fact that the nuclear substance 

 alone is the hereditary substance was furnished when it was found 

 possible to introduce into a non-nucleated piece of a mature ovum of 

 one species the nucleus of another related species, and when it was 

 seen that the larva that developed from the ovum so treated belonged 

 to the secoTid species. Boveri made this experiment with the ovum 

 and spermatozoon of two species of sea-urchin, and believed that he 

 had succeeded in getting from non-nucleated pieces of the ovum of 

 the first species, fertilized with the sperm of the second, larvae of this 

 second species; but, unfortunately, later control-experiments made 

 by several investigators, especially by Seeliger, have shown that this 

 result cannot be regarded as quite certain and indubitable. 



I must emphasize again that I am far from regarding the cell- 

 protoplasm of the ovum as an indifferent substance. It is certainly 

 not only important but indispensable for the development of the 

 embryo, and it has assuredly its own specific character, as in every 

 other kind of cell. It represents, so to speak, the matrix and nutritive 

 environment in which alone the hereditary substance can unfold its 

 wonderful powers ; it has developed historically, like every other kind 

 of cell, but it contains nothing more than the inherited qualities of 

 this one kind of cell-protoplasm, not those of the other cells of the 

 body. 



But although the essence of fertilization lies, as we have seen, in 

 the union of the hereditary substance of two individuals, and not 

 J. Y 



