226 INTRODUCTION TO CYTOLOGY 



the chromatic elements" (Hertwig 1890). But the chromatic mass is 

 actually quartered at reduction, whereas the number of chromosomes is 

 halved. Moreover, great changes in nuclear volume occur with no 

 change in the number of chromosomes. This careful guarding, so to 

 speak, of the chromosome number was siezed upon as a most significant 

 fact by Roux, who "argued that the facts of mitosis are only explicable 

 under the assumption that the chromatin is not a homogeneous substance, 

 but differs qualitatively in different regions of the nucleus; that the 

 collection 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 dis- 

 tributed to the daughter cells according to some definite law. He urged 

 that if the chromatin were qualitatively the same throughout the nucleus, 

 direct division would be as efficacious as indirect, and the complicated 

 apparatus of mitosis would be superfluous." 1 Upon this conception 

 Weismann based his remarkable theory, the starting point of which was 

 "the hypothesis of De Vries that the chromatin is a congeries or colony of 

 invisible self-propagating vital units or biophores, somewhat like Darwin's 

 'gemmules,' each of which has the power of determining the development 

 of a particular 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 ... 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 individual variations of the species, each chromosome therefore 

 being a particular group of slightly different germ-plasms and differing 

 qualitatively 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 'ancestral germ- 

 plasms' (ids) are arranged in a linear series in the spireme thread or the 

 chromosomes derived from it, Weismann ventured the prediction (1887) 

 that two kinds of mitosis would be found to occur. The first of these is 

 characterized b} r a longitudinal splitting of the thread, as in ordinary 

 cell-division, 'by means of which all the ancestral germ-plasms are 

 equally distributed in each of the daughter-nuclei after having been 

 divided into halves.' This form of division, which he called equal 

 division (Aequationstheilung), was then a known fact. The second 



^his and the following quotations are from Wilson (1900, pp. 245-246). 



