INTRODUCTION 7 



These microcysts lack a definite wall, and, on the return of 

 satisfactory conditions, revive and resume their activity. 



Both the swarmspores and myxamoebae, as they feed and 

 grow, normally multiply by repeated divisions. Finally they 

 are attracted toward each other, and tend to coalesce. In the 

 older literature multiple fusion of many such cells to form a 

 multinucleate mass is described. Cienkowski first applied the 

 term Plasmodium to the multinucleate naked thallus thus 

 formed. The character of the impulse which draws the cells 

 together was not understood, but the coalescence was believed to 

 be vegetative in character. 



Recently certain workers (Gilbert 1928 a, in Dictydiaethalium; 

 Skupienski 1928, in Didrjmium; Wilson and Cadman 1928, 

 in Reticularia) have regarded the fusion as in part sexual, and 

 have reported fusion of uninucleate cells (gametes) in pairs to 

 form zygotes as the first step in Plasmodium formation. After 

 the zygote is formed it may fuse with other gametes, with zoo- 

 spores, or with other zygotes. According to Gilbert and Skupien- 

 ski a plurinucleate thallus thus results. In Reticularia, according 

 to Wilson and Cadman, ciliate gametes fuse in pairs, myxamoe- 

 bae being unknown. The zygote thus formed acts as a center of 

 attraction toward which other ciliate haploid cells (gametes or 

 swarmspores) are drawn. These incoming cells enter the cy1;o- 

 plasm of the zygote and are completely digested there, the phenom- 

 enon being regarded as of the nature of ingestion rather than 

 coalescence. Finally the original pair of gametic nuclei fuse to 

 form the nucleus of the zygote, and it by repeated divisions 

 furnishes the nuclei for the growing thallus. If this account is 

 correct the mature thallus of this species is hardly a Plasmodium 

 in the sense of Cienkowski. Since the Plasmodium has, however, 

 so long been regarded as distinctive of the Myxomycetes veri- 

 fication of the account must precede its general acceptance. 

 Much additional work on the nuclear history of the group is 

 desirable. In any case, the plasmodium is characteristic in 

 aspect, and the phenomenon of multiple fusion, however inter- 

 preted, is admittedly confined to the slime moulds. 



The Plasmodium may multiply by fragmentation, or may 

 increase in size by fusion with other plasmodia or myxamoebae. 

 If conditions are unfavorable it may encyst temporarily to form 

 a sclerotium. Normally the plasmodium is an actively growing 

 body with the power of amoeboid movement. It is multinucle- 



