A Special Unit for Each Character 241 



it could extend over a long time ; in the latter it must take 

 place suddenly. In the first case the individual parts of 

 the nuclear threads could be mated one by one; in the 

 latter this would have to take place everywhere simultan- 

 eously. 



How this process comes about is self-evident when we 

 assume special units, special granules in the nuclear 

 threads, for the visible characters of the organisms. There 

 must be as many units in the nucleus, as a plant or animal 

 possesses individual characters. And this, of course, is 

 the rule for both pronuclei. In the condition of the short 

 and thick chromosomes these units lie crowded together. 

 This is a definite stage in cell-division ; the units, at least 

 those of the interior of the group, remain in a condition 

 of enforced rest. But as soon as cell-division is com- 

 pleted, the nuclear threads stretch ; they become quite long 

 and thin, and indeed so long that a large part, perhaps 

 most of them, possibly all of them, come to the surface. 

 At least stretched out in a row in this way, the granules 

 must then be arranged one after another, perhaps in the 

 threads themselves, perhaps in their finest ramifications. 

 Now they become active, and if, at this time, nuclear 

 threads of the paternal and the maternal pronuclei lie 

 together in pairs, every granule can enter into communion 

 with its corresponding unit in the other pronucleus. 



There is no reason to assume that the exceedingly fine 

 structure of the nuclei, which is so strikingly to the pur- 

 pose and yet so simple, should be limited to what is visible 

 to us at present. On the contrary everything points to 

 the probability that, in the internal structure also of the 

 nuclear threads this same serviceable rule must prevail. 

 The whole complicated process of nuclear division has 

 for its object the division of the two pronuclei in such a 



