THE GENERAL PHYSIOLOGY OF THE PROTOZOA 193 



In the second or alkaline period the vacuole enlarges rapidly, to 

 more than its original volume. The red colour produced by staining 

 with neutral-red disappears. The clumped food- mass breaks up into 

 smaller particles again. From the red-staining granules of the first 

 period deeply-staining spheres arise, homogeneous, refractile, and 

 apparently fluid (Nirenstein, 181). According to Khainsky (170'5), 

 the grains or droplets which are formed gather at the surface and 

 pass out into the endoplasm ; they represent the first products of 

 the assimilatory process in the vacuole, and their further chemical 

 transformation takes place in the endoplasm itself (compare the 

 refringent bodies formed in the process of digestion in acinetans, 

 p. 458) . According to Nirenstein, however, the spheres become smaller 

 and smaller, being reduced to tiny grains which vanish completely, 

 dissolved in the vacuole-contents. The vacuole now diminishes in 

 size a second time, and passes to the anal region, where it fuses with 

 other similar vacuoles, and is finally rejected from the anal pore. 



In other cases, however, no acid reaction has been demonstrated 

 in the vacuoles at any time, as, for example, in Actinosphcerium a 

 peculiarity which is perhaps to be correlated with the fact that in 

 this form the prey is killed when seized by the pseudopodia. It may 

 be supposed that the processes which, in Infusoria, etc., go on during 

 the first or acid period of the food- vacuole, take place in Actino- 

 sphcerium and some other forms before the vacuole is formed, in 

 which case the vacuole itself shows only the second or alkaline phase 

 of the digestion. 



According to Greenwood and Saunders (163), any ingested particles excite 

 the secretion of acid, but the true digestive vacuole is only formed under the 

 stimulus supplied by nutritive matter. Metalnikbff (179), however, found 

 that in the same individual some of the food- vacuoles are first acid and then 

 alkaline, while others are alkaline throughout in their reactions, and others 

 again, but rarely, show an acid reaction throughout ; he concludes that the 

 living cell has the capacity of adapting itself to the food supplied, and of 

 altering the properties of its digestive juices in accordance with its require- 

 ments. The process is perhaps comparable to the manner in which the blood- 

 cells produce different anti- bodies when brought into contact with different 

 pathogenic organisms or toxins. 



The variety of ferments that have been isolated from different 

 Protozoa also indicates that the digestion takes a different course 

 in different cases. In the plasmodia of Mycetozoa, a peptic ferment, 

 which when acidulated dissolves fibrin, has been isolated ; but since 

 the protoplasm of the plasmodium has a distinctly alkaline reaction, 

 it was thought by some that the ferment must be without function. 

 Metschnikoff (180) showed, however, that the food- vacuoles formed 

 in the plasmodium had a strongly acid reaction, in contrast to the 

 protoplasm, and thus demonstrated the function of the peptic 

 ferment in the digestion. In other cases tryptic ferments have been 

 isolated (" amoebodiastase," etc.). 



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