368 PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 



Kolliker opposes this idea and maintains that the " idioplasma " passes 

 into all cells, in which it divides in course of development. Step by 

 step from tlie embryonic Liyers to the tissues, the constructive proc- 

 esses are under the direction of the nuclei containing this hereditary 

 substance. It remains in every nucleus for along period unaltered, in 

 order to finally, here earlier, there later, impress its constructive forces. 

 In certain elements, as in blood corpuscles, epidermal scales, etc., it 

 disappears, as the last product of division. 



E. Hertwig takes a similar view. Since embryonic and adult cell 

 division is differential, there must be a form of differentiation in the 

 nucleus; but this does not consist in the total elimination of some 

 qualities and survival of others, nor of a reduction in mass. The 

 mass and the properties remain the same in every cell ; the differen- 

 tiation consists in the activity of certain elements in certain tissues. 

 Thus we may say with De Vries that different "pangene" may leave 

 the nucleus and enter the cell in different tissues; or with Niigeli, that 

 special '^micelhe" come into activity at certain points; in other words, 

 the potential of the nucleus is differently exerted. Here again we 

 have the idea of patent and latent hereditary elements, such as appear 

 in the entire individual upon a larger scale. 



This is one of the most interesting jn-oblems for future investigation, 

 but the direction of research will, I imagine, cover a larger area of 

 cell content than the nucleus, as we are now swinging back to regard 

 the extra-nuclear archoplasni as an important factor in the process. 



In the following paragraph Hertwig expressed his vie\v of nuclear 

 control and cytoplasmic differentiation : 



"As I saw in the transformation of the nucleus during fertilization 

 proof that it is the bearer of hereditary substance, I recognized a great 

 advance in the fact that the nucleus leaves in the same form in every 

 cell, and in its vesicular capsule is somewhat removed from the metamor- 

 phoses of the cells. As JSTiigeli spreads his idioplasm as a net- work 

 throughout the whole body, so, according to my theory, every body- 

 cell contained in its nucleus its quota of hereditary substance, Avhileits 

 specific histological peculiarities were to be regarded as its plasma 

 products. " 



2d. The next question is the fate of the maternal and paternal con- 

 tributions to the embryo. Here there is a wide difference of opinion. 

 On the one side Van Beneden is the leader of those who regard each 

 cell of the body as in a sense hermaphro<lite; as we have seen, his views 

 of maturati(m and the significance of the extension of the polar bodies 

 were colored by this theory, for he regarded the germ cells as hermaph- 

 rodite until one sex was eliminated. But now that the researches 

 of HertAvig have given the last blow to Van Beneden's theory, and it 

 follows that there can be no nmle and female chromasomes, there still 

 remains room for the analogous view that the maternal and imternal 

 chromasomes remain distinct throughout the course of development, 

 not as sexual elements, but as substances with the same racial and 



