312 LECTURES ON BIOLOGY 



cells must therefore contain the complete idioplasm in spite 

 of heterokinesis. If Weismann, in order to overcome this diffi- 

 culty, assumes that in cases of this kind the cells in question 

 have been invaded during the division by the active, isolated 

 determinants, and also by complete ids containing the entire 

 idioplasm (which remained, however, ineffective under normal 

 conditions, and only entered into the development and brought 

 their primary constituents to development under certain con- 

 ditions), he contradicts with that assumption his own hypothesis. 

 For in that case we should have to reckon, apart from homoeo- 

 kinetic and heterokinetic division, with a third mode of division 

 in which only one part of the ids is divided, while another part 

 is transmitted to the daughter-cells uncurtailed, as a ' reserve 

 army,' so to speak. In order to satisfy all possibilities, we should 

 have to conceive almost every cell furnished with such latent 

 additional germ-plasm. That the cells contain more * rudiments ' 

 than can usually reach development is proved by many patho- 

 logical formations, as, for instance, the occurrence of hair or 

 teeth in cysts. 



Further, as Hertwig and Driesch have shown, it is possible, 

 by artificially changing the shape of ova by slight pressure and 

 other means, greatly to influence the course of the division 

 process, ' or to throw about the first nucleus-generations in the 

 ovum-cavity like balls,' and yet see ova which have been thus 

 treated yield a normal developmental result. If now the single 

 nuclei were already ' determined ' in their developmental tendency 

 as the result of unequal division of the idioplasm, the ova that 

 have been submitted to pressure would necessarily develop into 

 the queerest malformations. We may therefore assume that 

 the nucleus still possesses all qualities, for only in this manner 

 can we explain its apparently illimitable mutability. 



In order to explain the normal course of development we 

 may well dispense with this very circumstantial mechanism 

 which would be necessary for the division of the germ-plasm into 

 its complement of determinants and their distribution. Even 

 when we conceive all cells to be supplied with the full hereditary 

 substance we can comprehend the cause of the processes of 

 transformation which take place during the development of the 

 embryo. It is true that the question why embryonic cells, 



