496 THE AMERICAN NATURALIST. [Vol. XXX IX. 



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 : — 



(i) According to Allen, Rosenberg, Berghs, and Grdgoire, 

 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. 



