414 MANUAL OF BACTERIOLOGY. 



by the retraction of this thread which thickens as it shortens. 

 The cells may pull apart and be drawn together again by this 

 thread of protoplasm several tim6s in the course of an hour. 

 Division of the nucleus is in some cases typically amniotic. 

 By dropping cover-glasses in fixing fluid at various stages of 

 the process certain stages can be studied. The encystment 

 mentioned above takes place sooner or later in all culture after 

 longer or sorter period of multiplication and independently 

 of the amount of moisture, but drying out hastens the proc- 

 ess. Encysted amebas transplanted to fresh culture-medium 

 usually begins to show signs of awakening activity in twenty- 

 four hours. These consist of agitation of the granules of the 

 protoplasm, and finally pulsation of the vacuole. If too much 

 of the old bacterial growth is transplanted on to the fresh 

 culture medium, the cysts may fail to germinate. No multi- 

 plication takes place within the cyst. The ameba leaves the 

 cyst behind as an empty shell. 



Spore Formation. — Spore formation in amebas takes place 



usually within 48-72 hours after they are transplanted to 



fresh medium; but it varies as. to time in different species. 



They first appear as fine, brightly refractive granules, which 



become larger and larger, by the aggregation of chromidia 



or by becoming surrounded by a layer of cytoplasm until they 



may become as large as the nucleus, and in fact have been 



mistaken by some observers for multiple nuclei. The spores 



vary in size from .7 to 2 mikra, and they stain faintly and 



are more readily decolorized than the amebte. They are 



extruded from the ameba in 24-48 hours from the time of 



their first appearance, but an ameba may become encysted 



before the spores are extruded which has probably led to 



the error of supposing that the encysted ameba may -form 



spores. The development of the spore into the ameba is 



direct; the thickening of the wall described by some does not 



take place if the conditions of growth are favorable. The 



