SPIROSTOMUM AMBIGUUM 399 



are present (PI. 22, fig. 2, vac). Occasionally as many as 

 five vacuoles have been seen in one large macrosome, each 

 vacuole being separated from those adjacent to it by strands 

 of the darkly staining substance of which the macrosome is 

 composed. 



The presence of small non-vacuolated granules, and medium 

 or large-sized ones containing one or more vacuoles, in the 

 same lobe of the nucleus is quite common. Sometimes all the 

 granules present in the nucleus are without vacuoles, whilst 

 in others they are all large with many vacuoles. 



Collin (6) describes similar macrosomes and microsomes in 

 the nuclei of Acinetaria. The microsomes he believes to be true 

 chromatin grains, whilst the macrosomes he regards as true 

 nucleoli. 



Owing to the fact that I have not had time or opportunity 

 as yet to study these structures in detail in Spirostomum, I 

 do not propose to offer any speculation as to their nature. 

 I should, however, like to add that since there exist all degrees 

 of vacuolation and non-vacuolation, and since whenever large 

 masses with numerous vacuoles are present the actual number 

 of masses is small, it seems to me very probable that the large 

 vacuolated masses (i. e. Greenwood's macrosomes) are formed 

 by a flowing together of a number of the smaller macrosomes 

 and a subsequent vacuolation from several centres. 



Animals having large multivacuolated macrosomes in their 

 meganuclei do not seem to be otherwise abnormal, and show 

 no signs of degeneration in the cytoplasmic structures. Since 

 the degree of vacuolation seems to be independent also of the 

 degree of growth after fission it seems quite probable, as Green- 

 wood suggests, that it is due to diet or to some temporary 

 condition of the culture medium. 



The M i c r o n u c 1 e i . — The micronuclei of S p i r o s t o m u m 

 ambiguum are minute in size and difficult to find. They 

 completely escaped the notice of Stein (28). Maupas (17) 

 was the first to discover their existence. They lie close to, 

 but are not attached to, the meganucleus (PI. 22, fig. 1, M.N.). 

 In structure they consist of a central endosome, presumably 



