142 COMPARATIVE ANATOMY 



pseudopods are manifestations of the contractility of the 

 protoplasm, and contractility is further exhibited by the 

 ectosarc ; this, when the animal is subjected to a weak electric 

 shock, a sudden jar, or other stimulus, may be seen to 

 contract, diminishing the size of the vacuoles, which, as 

 the effects of the stimulus pass away, recover their normal 

 dimensions. 



An Amoeba, as we have seen, possesses a single contractile 

 vacuole which is credited with excretory functions. Actino- 

 sphaerium has two or more contractile vacuoles, situated 

 superficially on the siirface of the ectosarc. They expand 

 slowly, swelling till they project like bubbles from the surface 

 of the body, and then their outer walls collapse inwards, 

 expelling their fluid contents, though no definite channels for 

 their escape can be observed. At times two or more 

 Actinosphaeria may be seen to come into contact, and their 

 pseudopodia fuse together. The process of fusion goes on 

 until their bodies come into contact and also fuse, so that 

 a double individual or a colony of several partially-fused 

 individuals is formed. This phenomenon has been called 

 plastogamy. Its significance is doubtful; no fusion of the 

 nuclei has been observed to accompany the fusion of the 

 ' cytoplasm of the cell-bodies, nor does the phenomenon appear 

 to be in any way connected with reproduction, so it cannot be 

 regarded as a process of conjugation, such as will be more 

 particularly described in other Protozoa. 



Normally Actinosphaerium reproduces itself by binary 

 division, and a celebrated observer of the life-histories of 

 Protozoa states that if a number of these animalcules are kept 

 without food for a time, and are then provided with a super- 

 abundance of food in the shape of the common ciliated 

 infusorian Stentor, they multiply rapidly by division. In 

 binary division the globular body is simply divided into two 

 halves by a constriction, each half containing a number of 

 nuclei. But both before and after division the number of 

 nuclei is constantly increasing by mitotic division. The 

 mitosis, however, differs from that which has been described 

 as typical for the cells of multi-cellular animals, and since it 

 may be regarded as a type of the modified form of mitosis 

 characteristic of Protozoa, it will be worth while to consider it 

 in some detail. 



