1916] Wilson: On tJie Life-History of a Soil Amoeba 247 



first case only slightly granular and close to the membrane, giving it 

 a thickened appearance, and in the second ease very granular and in a 

 looser layer. There is a clear zone between this and the single large 

 centrally located, spherical, compact karyosome, which may have .stain- 

 able lines extending outward from it to the peripheral chromatin in 

 a delicate network (pi. 20, figs. 39, 40, 44) . Sometimes its central region 

 stains very much less than the peripheral portion, in which case a dark 

 granule, the centriole, may sometimes be distinguished in or near its 

 center (pi. 20, fig. 59). 



3. BiN.MJY Fission 

 Division is promitotie, that is, with large chromatic polar ma.s.ses 

 coming from the karyosome, and spindle within the nuclear membrane 

 between these masses. The amoeba does not round up, but may move 

 about during most of the process. 



(a) Prophase 



The prophase is characterized by an increase in the size of the 

 nucleus, the diameter sometimes reaching seven microns (pi. 20, fig. 39). 

 The membrane is distinct, and the peripheral chromatin is more evident 

 than it is in the resting condition. The central karyosome, within 

 which a central granule or centriole may sometimes be distinguished, 

 increases in size, and then elongates. The peripheral chromatin 

 migrates in from the nuclear membrane towards the karj'osome, which 

 at this stage becomes "bent dumb-bell" in shape (pi. 20, fig. 47) and 

 the peripheral chromatin comes to lie in the angle. 



Spindle fibers are formed between the chromatic polar masses (pi. 

 18, figs. 2, 3), and an equatorial plate of chromosomes appears on 

 them (pi. 18, fig. 9), at which time the connection between the polar 

 masses or central spindle is within the spindle, that is, the chromosomes 

 are largely peripheral. Polar caps, which tal?e the plastin stain, may 

 be detected in the late stages of the prophase on the outer ends of the 

 chromatic masses (pi. 18, figs. 6, 7, 9, 10"). They are conical in shape 

 with the broad part of the cone resting on the chromatic polar nia.ss. 



(b) Metaphase 



Eight subequal chromosomes organized out of the chromatin 



granules in the angle between the parting polar chromatic masses 



appear in the equatorial plate. They are ellipsoidal, about twice as long 



as wide, and show no marked differentiation among themselves. When 



