304 LECTURES ON BIOLOGY 



smallest units, but in their turn contain an enormous number 

 of biophores which control still smaller parts of the body. Con- 

 cerning their size it can only be said that they lie far beyond 

 the limit of visibility, and that the minute granules revealed to 

 us in the germ-plasm by the strongest microscope represent 

 always groups of biophores. But these last ' heritable units ' 

 must be larger than any chemical molecule because they are 

 themselves constructed of groups of molecules. 



In the protozoa the determinants may probably consist of 

 single biophores, in which case biophores and determinants become 

 identical ; but in the higher organisms the determinants, as has 

 already been said, are composed of groups of biophores " which 

 joined together by inner forces form a higher life-unit. The 

 determinant must be able to live as a whole, i.e., be able to 

 assimilate, grow, and reproduce by fission, as every other life-unit, 

 and the biophores must be able to vary singly, so that also those 

 individual parts which they control are hereditarily variable." 

 But determinants and biophores are not distributed arbitrarily 

 but each id has a definite architecture. The hypothetical forces 

 which bind the determinants to their definite position in correla- 

 tion to the rest Weismann describes as ' vital affinities.' By means 

 of these different hypotheses Weismann now seeks to answer the 

 question as to the causes of the different formation of the cells 

 in the course of evolution. This brings us to the second and 

 more important part of his theory. 



Our observations of the process of mitosis impelled us to 

 the assumption that all cell-divisions lead to an equal division of 

 the hereditary substance among the descendants, so that each 

 body-cell contains an equal number of paternal and maternal 

 chromosomes and possesses the ' rudiments ' for all heritable 

 characteristics. But Weismann advances a diametrically opposed 

 opinion. He distinguishes sharply between two different kinds 

 of cell-division, homoeokinesis and heterokinesis (erbgleiche und 

 erbungleiche Zellteilung), which lead to very different results, 

 though, to judge from external appearances, they seem to 

 proceed on uniform lines. 



The first kind of reproduction is the usual, occurring in all 

 those cases where unicellular organisms divide by fission into 

 two equal daughter-organisms, or where the cells of the higher 



