496 THE AMERICAN NATURALIST. | [Vor. XXXIX. 
a fully organized paternal and maternal spirem as described by 
Allen. The identity of the sporophytic chromosomes becomes 
entirely lost, according to Strasburger's explanation of synapsis, 
and the chromatin granules (* Gamosomen ") may be variously 
distributed in the new set of bivalent chromosomes (** Zygoso- 
men"). These * Zygosomen” are a new creation in the cell. 
All of the other theories, on the other hand, preserve the mor- 
phological entity of the sporophyte chromosomes which are of 
course of maternal and paternal origin but allows their distri- 
bution in various ratios to one another during the first mitosis of 
sporogenesis. The chromosome, however, remains a fixed mor- 
phological structure from one generation to another. These 
are fundamental differences which have a vital bearing on the 
discussion of hybridization, which will follow shortly, since one 
of the most important features of the problems concerns the 
preservation of the relative purity of the germ plasm. 
The chief characteristics of the two theories of reduction 
may be summarized as follows : — 
(1) According to Allen, Rosenberg, eda and Grégoire, 
the phenomenon of synapsis presents a close association of two . 
parallel chromatic threads (probably of maternal and paternal 
origin) which finally unite to form the spirem that precedes the 
heterotypic mitosis. This single (fusion) spirem is then double 
in nature and the longitudinal fission which follows, is the sepa- 
ration of the two threads that entered into its composition. The 
reduced number of chromatic segments of the heterotypic 
mitosis are bivalent chromosomes or more precisely pairs of 
sporophytic chromosomes derived from the two (maternal and 
paternal) threads of the synapsis stage. The heterotypic mito- 
sis distributes the sporophytic chromosomes in two sets thus 
effecting a numerical reduction by one half. The sporophytic 
chromosomes divide prematurely during the heterotypic mitosis 
in preparation for the homotypic thus presenting a second longi- 
tudinal fission of the segments derived from the single (fusion) 
spirem. A special feature of Allen's studies is the fusion of 
chromomeres in pairs during the organization of the single 
(fusion) spirem and a subsequent splitting of each larger chro- 
momere with the longitudinal fission of this structure. 
