344 CLASSICS OF MODERN SCIENCE 

 nucleoplasm containing the hereditary tendencies of the nervous sys- 

 tem, and into that containing the tendencies of the external skin. 

 But even then, the end of the unequal division of nuclei would not 

 have been nearly reached ; for, in the formation of the nervous sys- 

 tem, the nuclear substance which contains the hereditary tendencies 

 of the sense-organs would, in the course of further cell-division, be 

 separated from that which contains the tendencies of the central 

 organs, and the same process would continue in the formation of all 

 single organs, and in the final development of the most minute histo- 

 logical elements. This process would take place in a definitely or- 

 dered course, exactly as it has taken place throughout a very long 

 series of ancestors ; and the determining and directing factor is simply 

 and solely the nuclear substance, the nucleoplasm, which possesses 

 such a molecular structure in the germ-cell that all such succeeding 

 stages of its molecular structure in future nuclei must necessarily 

 arise from it, as soon as the requisite external conditions are present. 

 This is almost the same conception of ontogenetic development as that 

 which has been held by embryologists who have not accepted the 

 doctrine of evolution : for we have only to transfer the primary cause 

 of development, from an unknown source within the organism, into 

 the nuclear substance, in order to make the views identical. 



I believe I have shown that theoretically hardly any objection 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, be- 

 cause 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 ex- 

 ist if it were possible for all somatic nuclei to contain a minute frac- 

 tion 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 



