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 havea varying 
structure according to the character and distribution of the ids 
which compose it. 
