442 INTRODUCTION TO CYTOLOGY 



biophore, a minute particle capable of growth and reproduction. The 

 many kinds of biophores in a given cell are the elements upon whose 

 presence the development of the cell's characters depends. The bio- 

 phores are grouped to form units of a higher order, known as determinants. 

 The determinant, since it is composed of the many kinds of biophores, has 

 the power of determining the development of a certain type of cell or 

 group of cells. In general, therefore, there are as many sorts of deter- 

 minants in the organism as there are types of cells, or "independently 

 variable parts." 



The determinants are, in turn, grouped into ids. A single id contains 

 all the kinds of determinants and so stands for the sum of all the charac- 

 ters of the organism. The ids in a given species differ only slightly 

 among themselves, the differences corresponding to the variations 

 observed within the species: they are the "ancestral germ-plasms" 

 contributed by past generations. The ids are identified with the visible 

 chromatic granules in the nuclear reticulum or in the chromatic thread 

 during mitosis. In most cases the ids are grouped to form idants, or 

 chromosomes. The id, rather than the chromosome, is the unit of 

 primary importance. The several ids in a chromosome are arranged in a 

 linear series, as suggested for the hereditary "qualities " by Roux (1883). 



Quoting Wilson (1900) again. 



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 chromo- 

 somes 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 by 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 hav ng been divided into 

 halves." This form of division, which he called equal division (^quat'ons- 

 theilung), was then a known fact. The second form, at that time a purely 

 theoretical postulate, he assumed to be of such a character that each daughter- 

 nucleus should receive only half the number of ancestral germ-plasms possessed 

 by the mother-nucleus. This he termed a reduction division (Reduktions- 

 theilung) and suggested that this might be effected either by a transverse 

 division of the chromosomes or by the elimination of entire chromosomes without 

 division. By either method the number of "ids" would be reduced; and Weis- 

 mann argued that such reducing divisions must be involved in the formation 

 of the polar bodies and in the parallel phenomena of spermatogenesis. 



With the aid of this elaborate mechanism Weismann explained onto- 

 genetic development in the following manner. In the zygote from which 

 the individual is to develop, all the kinds of determinants are present: 

 those of the female parent were contained in the egg nucleus and those 



