1923] Hall: Binary Fission of Menoidium incurvum 457 



BINARY FISSION 



Binary fission is initiated by division of the blepharoplast, rhizo- 

 plast, and granule, or extranuclear centrosome, at the base of the 

 rhizoplast ; later two flagella are seen. Whether the second flagellum 

 arises from the old by splitting, or as a now outgrowth from the 

 second blepharoplast with the old flagellum retained by the first, 

 could not be determined. Although outgrowth of the new flagellum 

 is to be expected, there is some indication of splitting beginning at 

 the blepharoplast and proceeding distally (pi. 40, figs. 10 and 12). 

 It is more probable, however, tljat this appearance is produced by 

 outgrowth of one flagellum in close conjunction with the other. 

 A similar case is described by Kofoid and Christiansen (1915&) 

 in Giardia, where the new flagella arise as outgrowths from the 

 blepharoplast, each new flagellum following the intracytoplasmic path 

 of the corresponding old one and emerging at its side. In Menoidium 

 the two flagella are at first closely associated, as shown in figure 14 

 (pi. 41), in which the two blepharoplasts are still close together and 

 the two flagella appear almost fused in most of their course through 

 the gullet. 



In Heteronema acus, Rhodes and Kirby (MSS) find that the new 

 flagella arise as outgrowths. Dobell (1908) states that in Copromonas 

 the old flagellum is drawn in on the approach of mitosis and, after 

 the division of the blepharoplast, a new flagellum grows out from 

 each daughter blepharoplast. Steuer (1904) records a splitting of 

 the flagella in Eutrcptia. Hartmann and Chagas (1910) describe 

 the outgrowth of a new flagellum from each daughter blepharoplast 

 in Peranema. Schiissler (1917) states that in Hcytomonas the old 

 flagellum may be retained by one daughter blepharoplast and the 

 other produced by outgrowth, or that both flagella may arise by 

 outgrowth as described by Dobell (1908). Thus, from the published 

 accounts it would seem possible that the new flagella may be formed 

 difl'erently in the various euglenoids, but this is rather improbable; 

 the origin of new flagella by outgrowth seems best supported. 



Several observers (Wager, 1899; Hamburger, 1911) have described 

 the flagella of various euglenoids as bifurcating, after entering the 

 reservoir, into two fibers, each of which ends in a basal granule or 

 blepharoplast. Haase (1910) figures the flagellum as bifurcating 

 at the level of the eyespot, and the resulting fibers as pa.ssing back- 



