REPRODUCTION AND HEREDITY 305 



organisms produce by fission cells of their own kind. In homoeo- 

 kinesis it is said that there takes place a doubling of the 

 determinants and an equal distribution among the separated 

 halves, respectively among the daughter-chromosomes ; but in 

 heterokinesis the determinants group themselves in the id 

 unequally, so that one id-h&lt contains determinants of one, 

 the other, determinants of another kind. This inequality of the 

 daughter-cells cannot be directly observed, as the determinants 

 lie beyond the limits of visibility, but * can only be concluded 

 from the different role which both daughter-cells play in the 

 farther development of the organism.' ' If, for instance, of two 

 sister-cells of the embryo one supplies the cells of the intestinal 

 canal, the other those of the skin and nervous system/ Weis- 

 mann concludes from that ' that the mother-cell has divided 

 its nuclear substance unequally among the two daughter-cells, 

 so that one received the determinants of the endoderm, the 

 other, those of the ectoderm ; or, if on a butterfly-wing a red 

 and black spot lie close together/ it follows according to Weis- 

 mann " that the stem-cells of these two spots divided unequally, so 

 that one received the 'red/ the other the 'black/ determinants.' 

 It appears from this that according to Weismann hetero- 

 kinesis plays the principal part in the development and trans- 

 formation of the ovum into the complete organism. Through 

 each division of the embryonic cells their germ-plasm suffers a 

 diminution in the number of determinants because the nuclei 

 of the daughter-cells receive always only those groups of 

 determinants which shall afterwards govern their own and their 

 descendants' formation and differentiation. In other words, 

 the germ-plasm of the fertilized egg-cell is divided during the 

 course of its ontogeny into more and more minute groups of 

 determinants, until finally the body-cells of the mature animal 

 possess only their own determinants. Nerve-cells possess, 

 therefore, only nerve-cell determinants, muscle-cells only muscle- 

 cell determinants ; hence they are still able to produce by fission 

 like, but no longer unlike, cells. In order that the embryonic 

 development may proceed normally, forces must be assumed in 

 the organism which watch with the greatest care that the deter- 

 minants always reproduce at the right moment, and always reach 

 at the right time the right place in the complete body. 

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