SOME MODERN VIEWS OF THE CELL. 613 



incidentally, to vegetative or non-sexual reproduction, which pre- 

 sents no further features of interest, since it differs from ordinary- 

 growth only in that the product of this form of growth does not 

 usually remain attached to, and form a part of, the parent indi- 

 vidual. On the contrary, it becomes separated, in most cases, 

 from the parent, and sets out as a new individual. But in sexual 

 reproduction we meet with a new complication. The phenomenon 

 of sexual union, which occurs, at least occasionally, in an enor- 

 mous majority of known organisms, and in very many must al- 

 ways precede reproduction, is essentially a fusion of two cells. 

 And, since the male cell often consists of little more than a nu- 

 cleus, it may perhaps be reduced, in its final expression, to a fusion 

 of two nuclei. Now it is observed that the number of chromo- 

 somes in a dividing nucleus of a given species of plant or animal 

 is approximately constant, and in the sexual nuclei quite so. 

 After a male sexual nucleus containing, for example, twelve 

 chromosomes has united with a female nucleus containing the 

 same number, the fertilization nucleus thus produced proceeds to 

 divide, and is seen to contain twenty-four chromosomes, or as 

 many as were brought to it by both parent nuclei. And this 

 number is found to persist without great variation in the nuclei 

 of the new organism developed from the fertilized cell by success- 

 ive divisions. It is plain that if the sexual elements produced 

 from organisms of this generation contained twenty- four chromo- 

 somes each, those of their sexually produced offspring would have 

 forty- eight each, and the point would soon be reached by success- 

 ive doublings at which the capacity of the nucleus would be far 

 overtaxed by the number of chromosomes. But this difficulty is 

 avoided, in the plants and animals thus far investigated, by an 

 abrupt reduction to one half the number usual in the organism, of 

 the chromosomes of the nuclei of certain cells which are to give 

 rise to the sexual cells. This reduced number remains constant 

 in all the descendants of the nuclei in which it first appears, until 

 the definitive sexual cells are formed. Then the fusion of two 

 nuclei, each with the half number of chromosomes, restores to the 

 resulting organism the typical number. This reduction has been 

 spoken of as abrupt ; and it could not well be more so. A nucleus 

 in which it occurs receives from its mother nucleus, let us say, 

 twenty-four chromosomes which fuse together to form its nuclear 

 network. When, after a period of rest, this nucleus proceeds to 

 divide, it develops from its network but twelve chromosomes, and 

 therefore furnishes but twelve to each of its daughter nuclei. 

 What has become of the other twelve no one can say, because 

 nothing is known of the exact relations that exist between the in- 

 dividual chromosome of the dividing nucleus and any part of the 

 network of the resting stage. 



