392 HARPER— ORGANIZATION, REPRODUCTION 



around it. This is a perfectly free swimming movement like that 

 at the corresponding stage in Hydrodictyon. Oltmanns ('04, p. 

 194) remarks that the zoospores probably remain connected by 

 threads of protoplasm but apparently has only Klebs' ('90) mistaken 

 contention as to the corresponding stages in Hydrodictyon as the 

 basis for the statement. A single spore can frequently be followed 

 clear through the mass or halfway round its periphery. 



This most active swarming period continues for three to four 

 minutes when conditions are favorable. It is followed by a rather 

 sudden slowing down and now the outlines of the future colony 

 appear. The sudden appearance of order out of the chaos of 

 swarming bodies is most striking. The circular outline of the 

 plate appears first and the peripheral cells seem to slow down in 

 their movements, while those in the interior are still quite active. 

 The free-swarming period thus passes over into a second and much 

 longer period of writhing and struggling in which the cells do not 

 move far from their places, but push this way and that between and 

 over each other, crowding and turning around and over without 

 getting completely out of connection with their neighbors as they 

 did in the earlier free-swimming stage. Coincidently with the slow- 

 ing down of the movements of the swarmspores they begin to take 

 on the four-lobed form of the adult cells. This change is very 

 conspicuous. The oval swarmspores seem as if they were about to 

 divide into two (Figs. 15-20). Each cell, as the figures show, 

 almost seems to be made up of two pear-shaped halves, the narrowed 

 ends of the halves being the future spines. This sudden assump- 

 tion by the swarmspores of the four-lobed form is a very striking 

 and conspicuous fact at this stage and is accompanied by rapid 

 growth and mutual pressure between the cells. Walls are formed 

 and the cell contours become more clear cut and definite. All these 

 changes begin with the slowing down of the movements of the 

 swarmspores. The process suggests very strongly the effort of the 

 cells to get into very specific relations with each other and as close 

 together as possible, thus forming the compact plate-shaped colony. 

 As the movement dies away, the behavior of individual cells can be 

 followed with more exactness. A change of position of one cell 



