1922] Kofoid^Swezy: Mitosis and Fission in Giardia enterica 215 



two blepharoplasts at the head of each axostyle ; in our material the 

 destaining was not carried far enough to detect the separate graunles, 

 if such exist. 



The Plane of Fission 



The plane of fission (pi. 24, fig. 12) is a longitudinal frontal one 

 rather than a sagittal one. In consequence of this the parent nuclei 

 of the right and left sides and their attendant neuromotor s.ystems 

 give rise by division or outgrowth to homologous parts of the daughters, 

 whereas if the organs of the two sides each gave rise to a daughter, 

 each lateral half of the neuromotor system must then necessarily give 

 rise to its supplemental half by a reversal of symmetry of the added 

 lialf, that is, the nucleus and neuromotor system of the right side 

 would at mitosis have one daughter cell retaining the parental 

 symmetry and the other assuming its mirror image. The telophase 

 figured (pi. 24, fig. 8) shows clearly that this does not take place, but 

 rather that the symmetry of the parental nucleus-neuromotor system 

 of the wliole organism is reproduced in the daughters by the frontal 

 division. 



The paradcsnwsc arises between the daughter centrosomes in the 

 prophase. The centrosome divides and one daughter migrates on the 

 outside of the nuclear membrane to the posterior pole of the nucleus, 

 spinning out a deeply staining strand, the paradesmose, between the 

 daughter centrosomes. This has no constant position, can be found 

 only when favorably located for observation (pi. 24, fig. 7), and soon 

 fades out as the metaphase passes. 



The Origin op Bilater.vl Symmetry in the Poly'mastigote 



PlAGELIoATES 



The polymastigote flagellates present two unique types of multi- 

 cellular somatellas, distinct in organization from the dendritic {Dino- 

 iryon), and radial (Symira) t.ypes evolved in the lower orders of the 

 Euflagellata. and from the geometrical types of plate (Gonium), 

 ellipsoid {Pandorina). sphere {Yolvox), and flattened, spirally bilateral 

 and anteroposteriorly differentiated ellipsoid (Platydorina), evolved 

 among the Volvocidea. 



These two unique types of somatellas are alike anteroposteriorly 

 differentiated in the location both of nuclei and of their attendant 

 neuromotor systems ; but in the number of nuclei and in their grouping 

 and in the interrelations of their attendant neuromotor systems, they 

 present two distinct types of symmetry, the spiral and bilateral. 



