80 



THE CELL 



FIG . 45. Mature human 

 spermatozoon from two 

 points of view. It is 

 composed of Jc head; m 

 middle portion ; and s 

 tail. 



function to perform. Firstly, they have to 

 keep the cell body afloat by means of their 

 activity, since its specific gravity is some- 

 what greater than that of the surrounding 

 medium. This is proved by the fact that 

 dead swarmspores and spermatozoa sink to 

 the bottom of the vessel. Secondly, they 

 have to propel the body in a certain direc- 

 tion by means of their movements. 



Nageli (III. 16) has made most careful 

 observations upon the mechanism of the move- 

 ments of the motile cells of plants. Accord- 

 ing to this investigator, the oscillations of 

 the flagella impart a two-fold movement of 

 the body a forward, and at the same time a rotatory movement. 

 Hence the resultant motion resembles that of a ball shot out of a 

 rifle. Such motions may be divided into three types : i 



" Many motile cells travel forwards in a straight or somewhat 

 curved line, the anterior and posterior ends of their axes remaining 

 exactly in the same direction ; these swim steadily forward, with- 

 out deviating from a fixed path. With others it may be distinctly 

 seen that they describe a straight, or somewhat bent spiral, in 

 which one revolution round the axis always corresponds to one 

 turn of the spiral (a given side of the cell always facing out- 

 wards), whilst the axis of the cell is parallel to that of the 

 spiral. Finally, there are other cells whose anterior ends describe 

 spirals, whilst their posterior ends proceed in a straight line, or 

 in a spiral of smaller diameter. The nature of the second and 

 third of these movements can only be distinguished if they are 

 very slow. If they are rather quicker, only a kind of wavering 

 can be made out, which, especially in the third, is of a peculiar 

 character." 



The direction in which the motile cells rotate about their 

 longitudinal axis generally remains constant for each kind, species, 

 or family; many rotate from south to west (Ulothrix) , others 

 from south to east (antherozoids of Ferns), others are somewhat 

 uncertain in their rotations, turning now from south to east, and 

 now from south to west (Gonium). If motile cells strike against 

 any object, they cease for a time their forward movements, but 

 continue to rotate about their longitudinal axes ; then, " as a rule, 

 they commence to retreat, their posterior ends being in advance, 



