108 University of California Publications in Zoology [Vol. 2o 



The further course of division of the ectoplasmic structures shows 

 a continued separation of the two portions until they come to lie on 

 opposite ends of the organism, connected by the spindle and para- 

 desmose (pi. 14, fig. 17). The relatively large portion of the body 

 which is not covered with ectoplasmic structures and flagella, renders 

 this separation very conspicuous in these stages. The completion of 

 these structures for each daughter cell is partly the result of new 

 outgrowths and partly the readjustments of those derived from the 

 parent cell. At the time plasmotomy occurs these may be completed 

 or may still be in the process of formation (pi. 14, figs. 24, 25). 



Mitosis 



The first evidences of the approach of mitosis may be looked for 

 in the nucleus. How early these appear cannot be stated, but, as in 

 the case of Trichonympha, the relative abundance of individuals show- 

 ing nuclear organization leading to the appearance of distinct chromo- 

 somes, would suggest that it begins soon after the completion of a 

 previous division period. 



Prophase: The chromatin of the nucleus may be disposed in 

 large granules (pi. 13, fig. 2) or in smaller granules with the network 

 connecting them thickly enci'usted with chromatin (fig. 4). In the 

 change from one condition to the other there seems to be a reduction 

 in the amount of chromatin. The nucleus, however, becomes enlarged 

 hence the reduction may be more apparent than real. The chromatin 

 of the granules continues to move out along the network until the 

 threads of the latter have become greatly thickened and the granules 

 have disappeared (pi. 13, fig. 5). This produces a coarse, heavj^ 

 network in which many free ends may be detected (fig. 12). These 

 are most frequently found at one side of the nucleus, with the opposite 

 side staining more densely and presenting an unbroken outline. A 

 further disintegration of the network is shown in figure 9 of plate 13. 

 Here its component parts are forming threads which begin to take on 

 the appearance of chromosomes on one side of the nucleus, wliile tlie 

 other still retains its network formation. 



The threads thus formed split longitudinally (pi. 13, fig. 6), while 

 at the same time becoming thicker, with the chromatin arranged in 

 distinct chromosomes. The outer alveolar zone of the nucleus dis- 

 appears, though this may frequently occur at an earlier stage. The 

 chromosomes, which at first are straight or irregularly twisted or bent 



