478 THE AMERICAN NATURALIST. [VOL. XXXIX. 



closed a few months ago. The chief points of issue in dis- 

 cussions of reduction phenomena have centered around the sig- 

 nificance of the heterotypic and homotypic mitoses. A typical 

 mitosis is believed to present merely a quantitative division of 

 each chromosome into two halves equivalent in their potentiali- 

 ties. The evidence for this view lies in the longitudinal fission 

 by which each chromatin granule on the spirem is supposed to 

 divide and contribute half of its substance to each daughter 

 chromosome. Can there be a qualitative division of a chromo- 

 some by which one of the parts differs in character from the 

 other, and are there such divisions at the time of sporogenesis 

 in plants and gametogenesis in animals when reduction phenom- 

 ena take place ? These have been the chief topics of dispute in 

 studies of this character for two decades. 



The problem then ultimately concerns the structure of the 

 chromosome and the reason for the constant reappearance of the 

 number characteristic of the species at the beginning of each 

 new gametophyte generation. All the prominent theories of 

 heredity assume that the chromosomes are made up of simpler 

 elements which stand for characteristics of the race. These 

 may form various combinations of higher orders and collectively 

 give the qualities of germ plasm. The simplest members that 

 can be observed in such a series of structures are the chromatin 

 granules (chromomeres) which may be found at almost all times 

 in the nucleus and are especially conspicuous when arranged in 

 a row on the linin thread of the spirem. Weismann has devel- 

 oped the most complex conception founded on the above princi- 

 ples and with the most elaborate terminology. Starting with 

 the chromatin granule, which he named an id, Weismann 

 assumed that this element is composed of still smaller structures 

 called determinants and biophores, the last being the ultimate 

 living units. Groups of ids make up idants or chromosomes. 

 The id was conceived to possess all the essential characters of 

 the specific germ plasm concerned but ids vary somewhat among 

 themselves, determining thus the individual variations of the 

 species. Therefore a chromosome or idant will have a varying 

 structure according to the character and distribution of the ids 

 which compose it. 



