390 HARPER— ORGANIZATION, REPRODUCTION 



the rounding up at the edges of the clefts resulting in open furrows 

 over the surface before the cell is completely cut through. 



In these plump-celled forms which are about to reproduce the 

 intercellular spaces are much reduced in area and their form is 

 somewhat changed (Figs. 9-14). The cells may, however, reach 

 maturity and divide without becoming so plump as those shown in 

 the cases just noted. In the stage shown in Fig. 13, cleavage is 

 complete and yet the cells are much more deeply lobed than those 

 in Fig. II, where sleavage has only reached the eight- to sixteen- 

 celled stages. 



The study of the living material is very convincing as to the 

 existence here of division by centripetal furrowing, with rectangular 

 intersection of the cleavage planes. The first cleavage plane is 

 regularly in the short axis of the cell (Fig. 10). The successive 

 planes of division cut each other approximately at right angles 

 through the four- and eight-celled stages. In the sixteen-celled and 

 thirty-two-celled stages there are modifications due to the irregular 

 outline of the mother cells. In general, however, the whole series 

 of divisions tends to illustrate the principle of rectangular inter- 

 section. 



These processes of division as a rule take place during the night, 

 as Smith has noted ('16), and swarming occurs at daylight, or a 

 little later in cool weather. This is also the case in Hydrodicfyon 

 ('08). By sealing up with hot paraffine an ordinary water mount 

 containing well-grown colonies of Pediastrum this rhythm may be 

 readily disturbed. It is probable that the accumulation of waste 

 products is an important factor in such cases. At any rate in prep- 

 arations sealed up on the previous evening colonies in various stages 

 of division may be found the next day. The division goes on much 

 more slowly under these conditions or may be inhibited all together. 

 I have kept cultures so sealed for long periods without visible de- 

 terioration of the colonies. The young colonies may continue to 

 grow for some time, but after the first day or two no cell division 

 occurs. The colonies may be kept alive for six months if they are 

 kept sealed and not allowed to dry out. 



The appearance of the mother cell after division is practically 



