PHYSIOLOGY OF AMCEBA. 163 



through the ectoplasm, and immediately afterwards disappears. 

 The contractile vacuole is almost certainly to be regarded as a 

 simple kind of excretory apparatus, the water which collects in 

 it containing in solution various products of destructive metabo- 

 lism which are thus passed out of the body.* 



Reproduction. However abundant the food-supply Amoeba 

 never grows beyond a certain maximum limit. After this limit 

 has been attained the animal sooner or later divides by "fission" 

 into two smaller Amoebae (Fig. 85, A). Thus the existence of 

 an individual Amoeba is normally terminated, not by death, but 

 by resolution into two new individuals. This process is the 

 simplest possible form of agamogenesis, and Amoeba is not known 

 to multiply in any other way.f The fission of Amoeba is a 

 process essentially of the same nature as the division of ordinary 

 tissue-cells, a division of the nucleus preceding that of the 

 cytoplasm. Whether the division of the nucleus is of the indi- 

 rect type (i.e. : passes through the phenomena of karyokinesis) is 

 not known by direct observation, but there is some reason to be- 

 lieve that it is so. In any case the successive fissions of Amoeba 

 are directly comparable with the successive cleavages of the egg 

 of a metazoon (p. 25). The progeny of the Amoeba, however, 

 separate and form independent individuals, while those of the egg- 

 cell remain intimately associated to form a single multi-cellular 

 individual. Morphologically, therefore, a metazoon is comparable 

 not with a single Amoeba, but with a multitude of Amwbce. 



Physiology. The possible simplicity of animal structure is 

 well shown in Amoeba, which is morphologically an animal re- 

 duced to its lowest terms. Its physiological operations are cor- 

 respondingly primitive and rudimentary ; and by an analysis of 

 them we may discover what is essential and fundamental in the 

 physiology of animals in general. A survey of the various activ- 

 ities of Amoeba shows that these may all be reduced to a f ew funda- 

 mental physiological properties of the protoplasm,^; as follows: 



* It may be recalled that the cavity of the nepliridiuin in the earthworm is 

 intra-cellular, like a vacuole (p. 60). 



f It has been asserted that Amoeba conjugates and also that it multiplies by 

 endogenous division ; but the evidence on both these points is inconclusive. 



J It is hardly necessary to remark that in common with all English-speak- 

 ing biologists we are indebted to Foster for the first comprehensive elaboration 

 of the " fundamental physiological properties " as exhibited by Amaha. 



