48 ENCYSTMENT AND EXCYSTATION OF AMCEBA 



one or more nuclei and occasionally a few vacuoles, 

 in which bacteria may sometimes be seen. Invest- 

 ing this closely is a thin layer, the " endocyst," and 

 in cysts which have only recently been formed there 

 may be nothing more visible. Gradually, however, 

 the "ectocyst" expands, and the protoplasmic 

 contents, with the endocyst encircling them, con- 

 tract. Thus in older cysts we find, starting from 

 the periphery, first the ectocyst, which is of con- 

 siderable thickness and is usually rather irregular 

 in outline, then a clear space, and finally the endo- 

 cyst, a resistant membrane which closely invests 

 the amoeba within. By means of the live slide 

 previously described, one can study the process 

 of encystment induced by choline, etc., and the 

 changes observed appear to throw considerable 

 light on its cause. The amoeba becomes more 

 sluggish and globular, and a clear halo of ecto- 

 plasm becomes visible at the periphery. Changes 

 which appear to be of a degenerative nature gradu- 

 ally occur in the ectoplasm, and the endoplasm 

 contracts and gets markedly denser, its outer 

 margin forming the resistant endocyst. Subse- 

 quently the thick irregular ectoplastic layer con- 

 tracts and becomes more hyaline, till it forms an 

 almost invisible layer closely investing the endo- 

 cyst. There appears to be a loss of water in 

 this process, as the resulting cysts are considerably 

 smaller than the amoeba? ; but we have no evidence 

 that it actually passes out of the cell, and osmosis 

 does not appear to us to play an essential part in 

 the process. Possibly the water is used up in some 

 chemical synthesis and is restored when excysta- 

 tion occurs. From our experiments it appears that 



