THE CHROMOSOME THEORY OF HEREDITY 291 



This theory was preceded by the hypothesis that the body 

 contained some substance which acted in a similar capacity. 

 This hypothetical sul)stance was called the idioplasm. The term 

 is reminiscent of Weismann's theories in which the terms id and 

 idant were prominent, Ijut before Weismann's contributions 

 appeared, Roux (1883) had recognized the similarity of chromatin 

 to idioplasm. Modern science has established a large body of facts 

 which are little short of actual proof that the chromosomes are the 

 material basis of heredity. 



The Organization of Chromosomes. During the resting stage 

 of a cell its chromatin is visible in stained preparations as a network 

 of uneven texture or as an aggregation of granules (Fig. 167). 



9 



d 



/I. /,«. 



Fig. 168. — Diagram of female and male diploid groups of chromosomes of 

 Drosophila melanogasler. The hook on the y chromosome is characteristic. 

 (From Morgan et al., Mechanism of Mendelian Heredity, with the permis- 

 sion of Henry Holt and Company.) 



This arrangement is lost when the nuclear membrane breaks down 

 at the beginning of cell reproduction and the chromatin condenses, 

 sometimes into a heavier thread which later breaks up into chi'omo- 

 somes and sometimes directly into chromosomes. In its condensed 

 state the chromatin appears to be a homogeneous material with a 

 marked affinity for many biological stains. 



According to cytologists, however, the chromatin is actually 

 composed of minute bodies called chromomeres which retain the 

 same relationships in all stages where they are visible. Wenrich 

 has shown that these chromatin units differ in size, so that their 

 arrangement in chromosomes can sometimes be traced readily. 



