No. 463.] STUDIES ON PLANT CELL.— VI. 489 



which are believed to have never actually fused during synapsis. 

 The reduced number of segments derived from the spirem pre- 

 ceding the heterotypic mitosis are then bivalent chromosomes 

 composed of pairs of sporophytic chromosomes lying side by 

 side. The heterotypic mitosis distributes the sporophytic 

 chromosomes in two sets resulting in a numerical reduction 

 of their numbers by one half. It will at once be noted that 

 while Berghs and Allen have independently arrived at similar 

 conclusions respecting the structure of the chromosomes of the 

 heterotypic mitosis there are some important differences in the 

 mode of origin. Allen reports an actual fusion of the two 

 threads (paternal and maternal) during synapsis and a later 

 fission of the spirem previous to the heterotypic mitosis. But 

 the accounts of both authors have much in common in their 

 interpretation of the structure of the spirem and chromosomes 

 of the heterotypic mitosis which is fundamentally different from 

 the accounts of Farmer and Moore, and Strasburger to be 

 described later. 



Gr6goire (:04) in a general discussion of reduction phenom- 

 ena confirms the observations of Berghs and takes a very posi- 

 tive position against the interpretations of Farmer and Moore 

 and Strasburger. The chief features of his conclusions are in 

 harmony with the results of Allen. The sporophytic (somatic) 

 chromosomes are believed to become associated in pairs by the 

 application of two delicate threads throughout their length during 

 synapsis. These threads are believed to retain their autonomy 

 and never actually to fuse although they may come in close con- 

 tact. Consequently the reduced number of chromosomes are 

 pairs of sporophytic chromosomes which have retained complete 

 independence. Allen, on the contrary, reports a complete union 

 of the two threads involving the fusion of chromomeres in pairs 

 and a later longitudinal division throughout its length of the 

 single (fusion) spirem. Gregoire does not regard the heterotypic 

 mitosis as a true nuclear division but as a special process designed 

 to effect this numerical separation of the sporophytic chromo- 

 somes and intercalated between typical mitoses, while Allen 

 would apparently treat it as a true mitosis and regard the chro- 

 mosome reduction as effected by the fusion of two sporophytic 

 spirems during synapsis. 



