1893.] Mode of Formation of Food Vacuoles in Infusoria. 471 



successive events in the digestive process in Carchesium, and, indeed, 

 the variability of many of them is marked. Changing conditions of 

 the animal, on the one hand (and some of these changes cannot easily 

 be controlled by the experimenter), and alteration in surroundings 

 which may be bound up with temperature, aeration, illumination, or 

 food, on the other, tend to bring out the elasticity of some of the 

 periods which I have distinguished. But this elasticity has limits, 

 nor does it characterise equally all the phases in the digestive cycle. 

 "We find that the total time of enclosure of innutritions matter may 

 be as short as 30 minutes, that nutritious substances have a mini- 

 mum (recorded) sojourn of 1 hour to 1J hour, and, on the other 

 hand, that the time of enclosure may be prolonged to 30 hours. 

 This great variation is found on examination to belong to that period 

 in the history of ingesta in which they are stored, inert and destitute 

 of fluid surroundings. The interval which separates successive acts 

 of ingestion in any one series varies from 30 sec. to 65 sec., and is 

 commonly 40 sec. ; the duration of the movement of progression varies 

 inversely (roughly speaking) with the duration of the phase of 

 quiescence. Thus, progression may occupy 5 sec. or 14f sec., but is 

 often lOf sec. ; quiescence, with a usual duration of 9 sec., may be 

 shortened to 5 sec., or lengthened to 25f sec. Aggregation is, as a rule, 

 instantaneous in vigorous animals, but f sec. or even f sec. have 

 passed between the onset and completion of the movement. The act 

 of solution is more variable ; I have seen very far reaching digestive 

 change in 50 min., but that variation should be more striking than 

 constancy is hardly surprising in face of the unlike nature of possibly 

 digestible matter. Lastly, as I have said, the stage of storage may be 

 omitted ; in this case digestion succeeds aggregation at once, the 

 fluid of the digestive vacuole increasing in amount and (presumably) 

 changing in composition ; on the other hand, ingesta may be stored 

 for 22 hours before they are attacked by the true digestive secretion. 



I have said above there seem to me to be grounds for regarding 

 the aggregation of ingested particles in this complete and vigorous 

 manner as a fundamental process in Protozoan digestion. Striking 

 as the phenomenon is in CarcJiesium, the actual displacement of 

 matter which it involves is, of course, small, and effective demonstra- 

 tion is possible chiefly because of the great transparency of the acting 

 cell substance, and because the food is naturally, or may be kept 

 artificially, in a state of minute division. Even in Carchesium, how- 

 ever, the marked retractility of the hyaline stalk of each polype often 

 hinders observation; and, when it is remembered that so many 

 Protozoa are vigorously motile, or relatively opaque, or deal (as do- 

 the Bhizopods) with comparatively massive food, it is hardly wonder- 

 ful that a secretion of matter which is (by virtue of its own 



