?M.d..(^ 



GENERAL OUTLINE 245 



developed certain ideas which afterward formed the foundation of 

 Weismann's whole theory of inheritance and development. Roux /^ 



argued that the facts of mitosis are only explicable under the assump- 

 tion that chromatin is not a uniform and homogeneous substance, but \\a^Y 

 differs quaUtatively in different regions of the nucleus ; that the col- ^ 



lection of the chromatin into a thread and its accurate division into 

 two halves is meaningless unless the chromatin in different regions of 

 the thread represents different qualities which are to be divided and 

 distributed to the daughter-cells according to some definite law. He 

 urged that if the chromatin were qualitatively the same throughout 

 the nucleus, direct division w^ould be as efficacious as indirect, and the 

 complicated apparatus of mitosis would be superfluous. Roux and 

 Weismann, each in his own way, subsequently elaborated this con- 

 ception to a complete theory of inheritance and development, but at 

 this point we may confine our attention to the views of Weismann. 

 The starting-point of his theory is the hypothesis of De Vries that the 

 chromatin is a congeries or colony of invisible self-propagating vital 

 units or biopJwres somewhat like Darwin's " gemmules " (p. 12), each 

 of which has the power of determining the development of a particu- 

 lar quality. Weismann conceives these units as aggregated to form 

 units of a higher order known as "determinants," which in turn are 

 grouped to form " ids," each of which, for reasons that need not here 

 be specified,^ is assumed to possess the complete architecture of the 

 germ-plasm characteristic of the species. The "ids" finally, which 

 are identified with the visible chromatin-granules, are arranged in 

 linear series to form " idants " or chromosomes. It is assumed further 

 that the " ids " differ slightly in a manner corresponding with the indi- 

 vidual variations of the species, each chromosome therefore being a 

 particular group of slightly different germ-plasms and differing quali- 

 tatively from all the others. 



We come now to the essence of Weismann's interpretation. The 

 end of fertilization is to produce new combinations of variations by 

 the mixture of different ids. Since, however, their number, like that 

 of the chromosomes which they form, is doubled by the union of two 

 germ-nuclei, an infinite complexity of the chromatin would soon arise 

 did not a periodic reduction occur. Assuming, then, that the " ances- 

 tral germ-plasms " (.ids) are arranged in a linear series in the spireme- 

 thread or the chromosomes derived from it, Weismann ventured the 

 prediction ('87) that two kinds of mitosis would be found to occur. 

 The first of these is characterized by a longitudinal splitting of the 

 thread, as in ordinary cell-division, " by means of which all the ances- 

 tral germ-plasms are equally distributed in each of the daughter-nuclei 

 after having been divided into halves." This form of division, which 



^ Cf. the Germ-plasm, p. 60. * 



