394 HARPER— ORGANIZATION, REPRODUCTION 



active mass of swarmspores but the plane of the picture is a httle 

 too high and the faintness of the swarming group is partly due to 

 this fact. Fig. 17 is from a plate exposed four or five minutes 

 later. The young colony lies partly beneath the second mother cell 

 but its outlines are in evidence and it is seen to be made up of 

 three concentric rings of cells like the other young colonies lying 

 about. Six of these sister colonies and parts of two more are shown 

 in this figure. The plane of the picture is a little below that of 

 Fig. 16 and other parts of the young colonies are in focus. I have 

 no positive proof of continued ciliary activity in colonies as old as 

 these, but none the less as one watches them they are seen to tip 

 from side to side and even shift their position slightly. Such move- 

 ments may well be due to slight currents in the water. 



As noted, these photographs cannot be regarded as successful, 

 tut they give a notion of the relative sizes of the mother cell and 

 the young colony at the time of its birth and the general relations 

 tinder which the morphogenetic processes take place. With the com- 

 plete cessation of the movements of its component cells it is obvious 

 at once that the organization of the future colony has been achieved. 

 With the establishment of the peripheral series the young spinous 

 projections are in general all pointing radially outward (Figs. 15- 

 20). In the same way in the inner series, generally speaking, the 

 two-lobed cells have the long axes of their lobes in the radii of the 

 colony viewed as a whole, just as in the full-grown colony. The 

 same exceptions and variations from these rules of orientation can 

 be found in such young colonies as are present in the older ones. 

 The relative number of cells in the outer and inner series, their 

 contacts and the shape of the intercellular spaces are all fixed as they 

 remain throughout subsequent growth and development. Rather 

 rapid growth continues for an hour or so, and with the increase in 

 size of the individual cells their mutual pressure and the develop- 

 ment of the intercellular spaces make the specific organization of 

 the colony more conspicuous. 



Summarizing, we may distinguish roughly five phases in the 

 vegetative reproduction .of Pcdiastrum. 



I. Nuclear division which goes on during the vegetative growth 

 of the cells and by which they become multinucleated. 



