GENUS AMCEBA—AMCEBA PKOTEUS. 43 



in the contractile vesicle, and all clear water-vacuoles of the endosarc, in 

 other Rhizopods and Protozoa in which these constituents occur. 



Sometimes two contractile vesicles are seen in Amoeba jproteus, as rep- 

 resented in fig. 6, pi: II. They occupy the usual position back of the. 

 nucleus, or may be temporarily separated by one shifting in advance of the 

 latter, or indeed both may be transferred to any other position. Rarely a 

 greater number of contractile vesicles occur, usually in such cases smaller 

 and often more or less scattered. . 



Sometimes after the disappearance of the contractile vesicle, two or 

 three minute ones appear simultaneously in its place, and, as they gradually 

 augment in size, break into one another until all become one. 



In the progressive movements of Amosba proteus, and the extension of 

 its pseudopods, the flow of the endosarc is accompanied with a more or 

 less thorough mingling of all its constituents. The smallest elements are 

 hurried along most briskly, while the largest exhibit more or less inertia ; 

 the nucleus and contractile vesicle almost always being hindmost in the race. 

 Occasionally, a more than ordinary impulse drives the nucleus in advance 

 of its habitual position, and even carries the contractile vesicle beyond it, 

 but in a little while they again assume their usual place. 



A remarkable fact in the streaming of the endosarc, with all its varied 

 constituents rolling among and jostling one another, is the circumstance 

 that the food-balls with their liquid envelope, the water-vacuoles, the con- 

 tractile vesicle, and all else, retain their integrity, as if they were solid, or 

 contained each within a membranous sac. Never are the materials observed 

 to break and run together, as a result of the continued jar to which they 

 are subjected. 



As previously intimaied. Dr. Wallich expresses the opinion that the 

 endosarc and ectosai'c of the Rhizopods are mutually convertible into one 

 another. When endosarc comes into contact with water, it undergoes a sort 

 of temporary coagulation, or conversion into ectosarc, and when the latter is 

 transferred into the interior after a time it again undergoes resolution into 

 the more liquid endosarc. In the taking of food he supposes that each por- 

 tion when swallowed becomes enveloped with a film of ectosarc, which forms 

 a vesicle enclosing the. food and water-drop in the interior of the endosarc. 

 As the food undergoes digestion, and the water, altered in condition, is im- 



