AND INHERITANCE IN PEDIASTRUM. 391 



complete is shown in Figs. 12, 13 and 14. Not all the division lines 

 can be brought out in the photograph. The mass is too thick and it 

 is plain that the swarmspores are in at least two layers. In these 

 reproductive stages, when the cells have become very turgid, and 

 especially during the stages of cell division, the mother colonies tend 

 to lose their flatness and become curved and bent in various ways 

 so that it is extremely difficult to get any large number of cells in 

 focus at once. Colonies with cells in which division is complete 

 when freshly mounted at daybreak are liable at any moment to show 

 the beginning of the swarming period. The cells of a colony rarely 

 swarm all at once. As a rule only part of the cells divide in any 

 one night and in the same way there is a succession in the initiation 

 of active movement in the mother cells, suggesting that internal as 

 well as external factors may be concerned in bringing about the 

 active swarming period. When once the swarm cells begin to move, 

 however, the succeeding stages are normally run through with great 

 speed and with no halting. The daughter cells first seem to round 

 up against each other and lose the rectangular outlines which have 

 been maintained by their mutual pressure. Slight twitchings and 

 quiverings can be seen in the mass whose significance is hard to 

 grasp but almost immediately the swarmers begin to glide upon one 

 another and show writhing, struggling movements. The mother cell 

 now bursts by the familiar crescent-shaped slit on its upper or under 

 surface and its contents slide out in the form of a sack or vesicle 

 containing the writhing mass of young swarmspores. This sack, 

 Al. Braun ('51) rightly observed, is the inner layer of the wall of 

 the mother cell. It is elastic and expands to over twice the diameter 

 of the mother cell as soon as it is free. In thus expanding, how- 

 ever, it still maintains the four-cornered outline of the mother cell, 

 as my photographs (Figs. 18-20) show. The two peripheral 

 spinous projections of the mother cell are still recognizable on it as 

 two symmetrically placed papillae. In the increased space provided 

 by the expansion of the vesicle the movements of the swarmspores 

 become much more active. The writhings become zigzag dashings 

 to and fro. The swarmspores shoot about in all directions through 

 the mass, which has become much looser, and into the open space 



