392 Louise Hoyt Gregory 



ole, but that the food particles are forced in the protoplasm, and 

 later are gathered into the true digestive vacuole where digestion 

 takes place in an acid medium. MetalnikofF ('03) has offered 

 the last suggestion in which the two opposing results are combined. 

 In his experiments on feeding paramecia with alizarin, he finds 

 the beginning of digestion taking place either in an acid medium 

 indicated by the yellow color of alizarin, or in an alkaline medium 

 indicated by the red color of alizarin, the main and final digestive 

 processes, however, always take place in an alkaline medium. 

 Thus the digestive processes of the Protozoa closely resemble 

 those of the Metazoa in which is found the pepsin ferment fol- 

 lowed by the pancreatin action. Attempts were made to duplicate 

 these experiments in case of Tillina, but in all cases the coloring 

 matter was not taken in. 



The endoplasm is well filled with large food vacuoles which are 

 in different stages of digestion. Some are crowded with bacteria, 

 and stain deeply with haematoxylin, indicating that digestion has 

 not proceeded far; some show lighter areas at the ends of the vacu- 

 oles, an indication of digestion in those regions; in others the proc- 

 ess has gone on to a greater extent, and only a slight amount 

 of undigested material remains, which is stained a pale gray in 

 comparison with the black stain of the fresh food particles; finally 

 vacuoles are found with no trace of the presence of food; these 

 can hardly be distinguished from the contractile vacuole, position 

 only being the means of identification. 



IV REPRODUCTION AND ENCYSTMENT 



Encystment is an expression of certain physiological conditions 

 in the cell during which different functions may be performed. 

 The cyst may be temporary only, and for the purpose of repro- 

 duction or digestion, or it may be permanent, affording the organ- 

 ism a condition of rest as well as a protection from an unfavorable 

 environment. The reproductive cysts are of two kinds, those in 

 which simple division takes place, and those in which so-called 

 sporulation occurs. 



Cohn, in 1853, described s mple division within a cyst, in the 

 case of Prorodon teres, one division only taking place. Carter, 



