558 LECTURE XXI. 



merely to this particular phenomenon. The throwing out 

 of a pseudopodium by an amoeboid zoospore, the formation 

 of a protuberance of the ectoplasm into which the more fluid 

 endoplasm is, as it were, sucked, is a phenomenon of con- 

 traction. So is also the lashing of a cilium whether, as some 

 suppose, the contraction takes place in the cilium itself, or, as 

 others suggest, in the cell bearing it. 



Little as we know about the mechanism of the loco- 

 motory movements of ciliated or amoeboid organisms, we 

 know even less as to the mechanism of the streaming-move- 

 ment of protoplasm. It appears to be allied to the amoeboid 

 movement, and, like that, to be a manifestation of con- 

 tractility. 



The phenomena presented by contractile vacuoles are of 

 great physiological interest, though their mechanism is im- 

 perfectly understood. With regard to the sudden systole, it 

 is not clear whether it is due to an active contraction of the 

 protoplasm, or merely to elastic recoil ; but it seems probable 

 that the former is the case, for Cohn has observed that the 

 vacuoles of Gonium and Chlamydomonas remain as clear 

 spaces when the zoospores are mounted in glycerin, under 

 conditions, that is, which render any pressure of the con- 

 tained liquid on the wall impossible. 



We come now to the movements of organs consisting of 

 one or more cells clothed with a cell-wall. The structure of 

 the cell or cells concerned in the movement is that which has 

 already been described more than once : there is, namely, an 

 extensible and elastic cell-wall, lined by a layer of proto- 

 plasm, enclosing a vacuole filled with cell-sap, and we have 

 to ascertain how a movement can be performed by such an 

 organ. 



We will begin with instances of movement which admit 

 of direct observation, and will then pass to others which, 

 from the nature of the case, cannot be so readily investi- 

 gated. Such instances are afforded by the movements per- 

 formed by mature organs in response to a mechanical or 

 electrical stimulus. It was pointed out in the last lecture 

 (p. 545) that the effect of stimulation on the stamens of the 



