ioo AMEBOID MOVEMENT 



both organisms is essentially the same, so far as could be de- 

 tected. In both organisms the energy of streaming is liberated 

 within the endoplasm. This is especially well shown in the first 

 stages of feeding. 



Besides these organisms in which streaming occurs, either in 

 a part of the organism or the whole, streaming is also found 

 to occur in a great variety of plants other than those already men- 

 tioned ; in the leukocytes of perhaps all coelomates ; in some 

 animal egg cells, such as the sponges, hydra and molluscs; in 

 pigment cells, especially in batrachians and lacertilians ; in phago- 

 cytes and wandering cells of a great many animals ; in the nuclei 

 of some animal cells ; and in the intestinal epithelial cells of per- 

 haps all metazoans. In almost none of these cases however do 

 we know more than the bare fact that streaming occurs. No 

 details are known. Consequently in so far as the purposes of this 

 book are concerned it will not be apropos to discuss these cases 

 further except to record the thesis that there is no evidence tend- 

 ing to show that these cases are not at bottom all characterized 

 by the operation of the same fundamental process. 



In all these cases of animal and plant cells and tissues in 

 which ameboid movement occurs the process of streaming is 

 easily observed in all of them, but the phenomenon of contractility 

 is not .noticeable in some cells except under special conditions, 

 while in other cells it is operating continually. This indicates 

 that there are other factors at work in addition to mere phase 

 changes in the colloidal system to produce now contractility, now 

 streaming. A high power of contractility and of streaming are 

 not present in the same mass of protoplasm at the same time, 

 though these powers may both be present at different times 

 (Biomyxa). 



Contractility can be explained in a general (though not yet in 

 a detailed) way as due to a change in phase, more or less com- 

 plete, in the colloidal system which is held to be the chief char- 

 acteristic of the physical aspect of protoplasm. The change of 

 phase is of course, associated with a change in the amount of 

 surface energy, which is the ultimate source of the energy of 

 contractility. 



Streaming, however, does not depend upon a marked change of 



