492 THE AMERICAN NATURALIST. [VOL. XXXIX. 



The spirem thread thus breaks up into segments which, how-- 

 ever, lie in pairs represented by the V's in the reduced (gameto- 

 phyte) number. The pairs are bivalent chromosomes, each 

 composed of two sporophytic chromosomes which were arranged 

 serially on a single spirem thread. The pairs are not always 

 organized through the approximation of the arms of V-shaped 

 loops but this is a very characteristic type of structure. The 

 V's have been interpreted by other authors as the approximation 

 of portions of the spirem thread (Dixon, '95, '96, : oo) or the 

 separation of their free ends at the bend of the loop as a trans- 

 verse division of a reduced number of looped chromosomes in 

 the heterotypic mitosis (Schaffner, '97). The two parts of the 

 bivalent chromosomes (which are pairs of somatic chromosomes) 

 now become shorter and thicker and all trace of the original 

 fission of the spirem thread is lost. 



The essential features of Farmer and Moore's interpretation 

 of the prophase of the heterotypic mitosis are, then : (i) a sin- 

 gle spirem with the sporophytic chromosomes arranged serially, 

 which splits only once longitudinally, the fission afterward 

 becoming obliterated when the chromosomes are organized, 

 and (2) the organization of bivalent chromosomes in the reduced 

 number largely by the approximation of the free ends of loops 

 which entails a separation at the bend of the loops of the two 

 sporophytic chromosomes, giving the appearance of a transverse 

 division. 



The heterotypic mitosis, then, according to Farmer and 

 Moore involves merely the distribution of the sporophytic chro- 

 mosomes arranged in pairs (bivalent chromosomes) as univalent 

 elements to each daughter nucleus. This is of course the gen- 

 eral conclusion of all recent investigators, the different views 

 being the result of varying accounts of the method of organiza- 

 tion of the bivalent chromosomes. During this distribution in 

 the heterotypic mitosis the split of the original spirem appears 

 in each univalent element (sporophytic chromosome) and the 

 halves open throughout the greater part of their length giving 

 the peculiar V-shaped daughter chromosomes so characteristic 

 of this mitosis in the lily. The arms of these V's become the 

 daughter chromosomes of the homotypic mitosis which are thus 



