1916] Wilson: On the Life-History of a Soil Amoeba 249 



(/■) Cytoplasmic Division 

 The amoeba may become elongated in the direction of the long 

 axis of the spindle in the early anaphase (pi. 18, figs. 10, 13) and 

 always does so in the late anaphase (pi. 18, fig.s. 8, 14). It begins to 

 constrict before the connection between the two nuclei is lost (pi. 18, 

 fig. 15). When it has become very slender, the daughter amoebas put 

 out pseudopodia (pi. 18, figs. 12, 15) and pull apart. The separation 

 is evidently accomplished with no little force from the contraction of 

 the released ends which may be observed in the living material. After 

 separation the cytoplasm immediately rounds off so that tliere is no 

 evidence of the point at which the separation finally took place (pi. 19. 

 fig. 25). However, the remains of the nuclear connection, comparable 

 to the achromatic strands or interzonal fibers leading to a cell plate, 

 persist for some time (pi. 19. fig. 25: pi. 20, fig. 43). 



(g) The Behavior of Organelles in Mitosis 



The organelles do not behave in exactly the .same manner in every 

 dividing individual, and while the brief account of mitosis as given 

 above describes the process in general, it will be well to add a dis- 

 cussion of the more minute structure and variable behavior of the 

 organelles during mitosis. 



The karyosome, averaging 1.5^ in diameter, often increa.ses to 2.5 

 and even 4/x prior to division (pi. 20, figs. 38, 39). It is very dark 

 at the periphery and light in the pla.stin center, which may be the 

 equivalent of the centrodesmose in the restricted sense of Heidenhain 

 (1907), appearing almost vacuolated in both the living and stained 

 material. There may be one or several of these "vacuoles" (pi. 18, 

 figs. 1, 4; pi. 20, figs. 39, 44). This vacuolated condition may persist 

 to late prophase (pi. 18. fig. 3), but not in all individuals (pi. 20, 

 figs. 47, 60). 



The karyosome is not granular until in the late prophase (pi. 18, 

 figs. 1-5. 9: pi. 20, figs. 37-39, 41, 55, 56), and even then it is not in- 

 variably so (pi. 20, figs. 47, 60). Also in all the later stages of division 

 it may be more or less granular, though often homogeneous in appear- 

 ance. When a granular condition is found, the slender coimection 

 between the two migrating chromatic polar masses may also be some- 

 what granular (pi. 20, fig. 55). This is not true when the connection 

 has become vei*y slender (pi. 18, fig. 9). 



The karyosome elongates, bends in on one side (pi. 18, fig. 1; pi. 20, 

 figs. 39, 45), and the ends round up so that it is .shaped like a bent 

 dumb-bell. The connection between the two ends gradually diminishes 



