Responsiveness in Single Cells - 1 97 



Fig. 11-7. Diagram illustrating move- 

 ment of a cilium. The cilium appears 

 to be relatively rigid during the ef- 

 fective stroke, but limp during the 

 recovery. 



EFFECTIVE STROKE 



RECOVERY STROKF 



Amoeboid movement is not restricted to 

 the amoeba and other protozoans, but is also 

 encountered in the white blood cells (leu- 

 cocytes) and connective tissue cells (fibro- 

 blasts) of higher animals. Leucocytes migrate 

 throughout the body by amoeboid move- 

 ment, squeezing their way among the cells 

 of other tissues, and engulfing particles of 

 organic debris and bacteria, if the tissue is 

 infected. This activity of leucocytes is called 

 phagocytosis, although essentially phagocyto- 

 sis resembles the process of ingestion in the 

 amoeba and other free-living cells. In the 

 human body there are also certain epithelial 

 cells, located especially in the liver and 

 spleen, which act as stationary phagocytes. 

 Despite the fact that these cells are anchored 

 in a fixed position in an epithelial surface, 

 each is able to thrust forth pseudopodia from 

 its free end and to engulf solid particles from 

 the blood stream, upon which the epithelium 

 borders. 



By watching a pseudopodium as it extends 

 from the surface of an amoeba, one can gain 

 some concept of how the movement is 

 effected. Not all the protoplasm flows as the 

 pseudopodium extends. In fact, there is a 

 semisolid layer of protoplasm — the plasma- 

 gel — which completely encases the fluid 

 (flowing) protoplasm, or plasmasol (Fig. 11- 

 9). Many experiments indicate that gelated 

 protoplasm possesses a capacity to contract, 

 and the plasmagel of the amoeba probably 

 contracts, exerting a pressure upon the plas- 

 masol and causing it to flow forward. At 

 the tip of an advancing pseudopodium the 

 plasmagel is virtually absent, so that the for- 

 ward flow is impeded mainly by the elastic 

 surface membrane. Moreover, the sol, upon 

 reaching the tip ol a pseudopodium, is di- 

 verted toward the side walls, where gelation 

 occurs. Thus the plasmagel oi the side walls 

 is built up as a pseudopodium extends. 

 Simultaneously, near the posterior extremity 



Fig. 11-8. Cross-sectional view of a 

 group of cilia (six) from the lateral sur- 

 face of the gill of a clam (Anodonta 

 caiaracta), photographed under the 

 electron microscope at a magnification 

 of 110,000 diameters. In each cilium, 

 note the nine double filaments, in cir- 

 cular arrangement just inside the 

 boundary membrane, and the one 

 double filament, at the center of the 

 cilium. (Courtesy of J. R. Gibbons, Har- 

 vard University.) 



