H. MUNRO FOX 



485 



experiments, one central aggregation is usually formed but some- 

 times there are several, always near the center. 



After an aggregation has been in existence for a short time a clear 

 space free from flagellates appears at its center. The central space 

 enlarges until the flagellates come to lie in a circular band around it, 

 this band being easily visible to the naked eye (Fig. 3) . The increase 

 in size of the clear area goes on steadily, and as the band of flagellates 

 surrounding it approaches the edges of the cover-slip the sides of the 

 band become flattened (Fig. 4) and then, still enlarging, the band 

 gradually comes to form a square, the sides of which are parallel to 



Fig. 3. Appearance of region free from flagellates in the center of the aggre- 

 gation. 



Fig. 4. Circular band of flagellates becoming square. 



the sides of the cover-glass. When this square-shaped band of flagel- 

 lates has reached a certain distance from the edges of the preparation, 

 it becomes stationary, the central clear area no longer increasing in 

 size (Fig. 5). The distance of the final position of the band from 

 the edge of the cover-slip depends on the height of the latter above 

 the slide; the band comes to lie the nearer to the edge the lower 

 the cover-slip. The great majority of the flagellates present in the 

 preparation are in the band, but nevertheless there are always a 

 few swimming in the region between the band and the edge of the 

 cover-glass. 



