THE VITAL PROPERTIES OF THE CELL 81 



and to rotate themselves in an opposite direction." This backward 

 movement never lasts for long, and is always slower than the 

 forward one ; however, the cell soon returns to its normal mode 

 of progression, which usually takes place in a somewhat oblique 

 direction. 



In consequence of his investigations, Nageli is of opinion that 

 if zoospores and spermatozoa be quite regular in form, if their 

 substance be evenly distributed throughout their mass, and 

 further, if the medium be quite homogeneous, they must travel 

 in a perfectly straight line, and hence that all deviations from this 

 straight line, both as regards rotation round the axis and forward 

 progression, must be ascribed either to the circumstance that they 

 are not symmetrical in form, and that their centres of gravity 

 are not in the centres of their bodies; or to the fact that the 

 frictional opposition which they encounter is not equal in every 

 direction. 



By means of flagella a far greater speed is attainable than by 

 means of pseudopodia. According to Nageli, zoospores usually 

 proceed at the rate of one foot per hour ; the quickest, however, 

 take only a quarter of an hour to traverse the ame space ; whilst 

 a man, at ordinary speed, traverses a distance of rather more than 

 half his length in a second, a swarmspore in the same time 

 covers a distance of nearly three times its own diameter. How- 

 ever, although the rate at which they move appears, when they 

 are seen under the microscope, to be very great, we must take 

 into account the fact that the distance is also magnified, and that 

 in consequence they appear to move much more rapidly than they 

 do in reality. As a matter of fact, their movements are exceed- 

 ingly slow. "Without magnification, even if the organisms could 

 be plainly seen, no movement could be perceived on account of its 

 slowness." 



Spermatozoa (Fig. 45) may be distinguished from the zoospores 

 of plants by their possessing one single thread-like flagellum, 

 situated at the posterior end of the body. The spermatozoon, 

 being propelled by it, advances by means of snake-like move- 

 ments, resembling those of many fishes. In some cases the 

 structure is more complicated, a delicate contractile or undulatiwj 

 membratie, which may be compared to the edge of a fish's fin, 

 being present. This is especially well developed on the posterior 

 end of the large spermatozoa of the Salamander and the Triton 

 (Fig. 46). 



G 



