THE INFUSORIA 



375 



entirely disconnected from the network only occur in a few 

 forms. 



In Spirocliona there is a remarkable arrangement of the elements 

 of the meganucleus, the chromatin being collected into a thin, saucer- 

 shaped mass, leaving a spherical space of clear achromatin in which 

 during the resting-stage a large deeply-staining spherule occurs (Fig. 

 22). The nature of this granule is very uncertain. By Balbiani 

 it was regarded as combining the characters of the centrosome and 

 nucleolus of the Metazoan cell. Before the division of the mega- 

 nucleus (Fig. 23) it disappears, and cannot be traced again until 

 after the separation of the daughter meganuclei. In this respect 



FIG. 22. 



Spirochonageinmipara, Stein, from 

 a stained preparation, to show the 

 meganucleus (M) in a state of rest 

 and one micronucleus (m). Size 

 about O'Oo mm. in length. (Original.) 



FIG. 23. 



Spirockona gemmipara during the 

 formation of a gemmula, showing 

 that, during the division of the 

 meganucleus, the nucleolar body is 

 absent. (From an original prepara- 

 tion.) 



it undoubtedly claims to be ranked in the category of nucleolar 

 structures ; but its claim to rank as a centrosome as well is, for 

 many reasons, unsatisfactory. 



It is still an open question whether any of the meganuclei are 

 surrounded by a definite membrane of a distinct texture. The 

 appearance of a membrana limitans, which may be seen in all 

 stained specimens mounted whole, is not always seen when the 

 meganuclei are cut into thin sections, and it may therefore be 

 accounted for as an optical effect due to the difference in density 

 between the nucleoplasm and the surrounding cytoplasm. This 

 explanation, however, will not account for the facts observed in 

 all cases, and it seems to be certain that a membrane of a distinct 

 chemical character is formed between the nucleoplasm and cyto- 



