1923] Hall: Binary Fission of Menoidium incurvum 451 



are clumped together at either one or both ends of the animal (pi. 40, 

 figs. 1 and 6), usually only at the anterior end and never altogether 

 at the posterior end; in no case have they been observed to fill the 

 entire body of the flagellate. 



The Neuromotor System 



Since the introduction of the conception of a neuromotor system 

 (Kofoid and Christiansen, 1915cr., h, Kofoid, 1916), subsequent investi- 

 gations (Swezy, 1916; Boeck, 1917 ; Kofoid and Swezy, 1919a, 6, 1920, 

 1922) have demonstrated the presence of such a system in many differ- 

 ent flagellates. The neuromotor system consists essentially of the 

 flagellum, the blepharoplast with its connected structures, and the 

 rhizoplast extending from the nucleus to the blepharoplast, thus form- 

 ing an integrated system composed of the nucleus and locomotor 

 organelles. In addition there is, at the base of the nuclear rhizopla.st, 

 an extranuclear centrosome, the center of the system. 



The system in Menoidium is simple : there is a single flagellum 

 ending in a blepharoplast, from which a rhizoplast extends to the 

 nucleus (fig. A; pi. 40, fig. 9). The blepharoplast is located at the 

 bottom or side of a vacuole-like enlargement, the reservoir, at the 

 posterior end of the gullet (fig. A). The nuclear rhizoplast is very 

 delicate and can be seen in well-stained material only ; Bordeaux red- 

 iron haematoxylin preparations are most favorable for such obser- 

 vations. So far it has been impossible to trace the rhizoplast in 

 flagellates in which the nucleus is at the posterior end. In many 

 forms, other than euglenoids, there is an extranuclear centrosome 

 at the base of the rhizoplast. In Euglena agilis (fig. B) a similarly 

 located granule is easily detected in Bordeaux red-iron haematoxylin 

 preparations; in Menoidium (fig. A) such a granule has been deter- 

 mined in similar preparations by the use of green-filtered mono- 

 chromatic light ; however, this granule in Menoidium has been found 

 in comparatively few of the flagellates examined, since it is very small 

 and does not always stain heavily. Thus it appears that the neuro- 

 motor system of at least some of the euglenoids is essentially similar 

 to that of Giardia (Kofoid and Swezy, 1922), Trichomonas (Kofoid 

 and Swezy, 1915a) and related forms, in that they possess an extra- 

 nuclear centrosome at the base of the rhizoplast. Structures such as 

 those described by Wager (1899) at the level of the eyespot in Euglena 

 are not present in Menoidium. 



