No. 463.] STUDIES ON PLANT CELL.— VI. 493 



formed prematurely during the heterotypic as was first described 

 by Gregoire ('99). However, Grdgoire and most botanists have 

 considered the split between the V's as a second longitudinal 

 fission of the original spirem in the spore mother-cell while 

 Farmer and Moore regard it as the reappearance of an original 

 single fission. This view of Gregoire, which has had the sup- 

 port of Guignard ('99), Strasburger (: 00), and Mottier (: 03), is 

 the theory of a double longitudinal splitting of the chromosomes 

 previous to the heterotypic mitosis and is also maintained in 

 Allen's (: 05) recent paper. 



The homotypic mitosis brings about the final separation of the 

 arms of the V-shaped longitudinally split univalent (sporophytic) 

 chromosomes of the heterotypic division^ The fact that the 

 arms of these V's finally break apart at the ends does not con- 

 stitute a transverse division as has been claimed by some earlier 

 writers (Ishikawa, '97 ; Calkins, '97 ; Belajeff, '98 ; Atkinson, 

 '99, for Trillium). The peculiarities of the homotypic mitosis 

 are then due to the premature fission of the univalent chromo- 

 somes during the heterotypic. As a type of nuclear division the 

 homotypic mitosis is not fundamentally different from the typi- 

 cal divisions of other periods of the life history. All recent 

 authors are in agreement on this interpretation of the events of 

 the homotypic mitosis. 



Gregory (: 04) gives an account of sporogenesis for several 

 leptosporangiate ferns and accepts Farmer and Moore's explana- 

 tion of reduction phenomena. He finds the same sort of U- 

 shaped segments in the reduced number at the heterotypic 

 division and considers them bivalent chromosomes which divide 

 transversely so that the original sporophyte chromosomes are 

 distributed in two sets during this mitosis. The various posi- 

 tions assumed by the limbs of the U-shaped segments give 

 appearances very similar to the tetrads described in the hetero- 

 typic mitosis of animals and which Calkins ('97) reported for 

 Pteris and Adiantum and regarded as resulting from the trans- 

 verse division of the halves of a longitudinally split chromosome. 

 Gregory of course cannot accept the conclusions of Calkins. 



Williams (:04a) apphes the theory of Farmer and Moore 

 respecting the bivalent character of the chromosomes in the 



