THE RAPIDITY OF MOVEMENT OF SWARM-SPORES, ETC. 607 



of them the anterior end (hyaline and provided with vibrating cilia) as well as 

 the posterior green end of the axis of the body remains exactly in the path of 

 progressive motion, be this straight or curved — ^they swim forwards rigid, and without 

 deviations. Others describe a direct or more or less curved spiral line, a rotation 

 on the axis of the body always corresponding to a spiral revolution, so that the same 

 side of the cell is always turned outwards while the axis of the body runs parallel with 

 the axis of the spiral path. Thirdly, there are swarmers the anterior end of which 

 describes a spiral line, while the posterior end describes a straight one, or a spiral 

 with a smaller diameter. These forms of motion are only to be detected when the 

 rotation and progression are slow. The motions of spermatozoids essentially agree 

 with these, according to Naegeli, and he is convinced that if the form were regular 

 and the mass distributed symmetrically, the cells would swim in a direct line in a 

 homogeneous medium, and that all deviations from this and the simple rotation round 

 the proper axis of the body depend upon the fact that the moving bodies are not sym- 

 metrically constructed, that their centres of gravity are not in the centre, and that they 

 do not experience equal friction all round. The direction of the rotation is usually 

 constant for each species, genus, or family ; nevertheless there are exceptions, as in 

 the tablets of Gomum. It is often impossible, however, to convince one's-self of the 

 direction of rotation in the case of unicellular swarmers ; and this, as Naegeli shows, 

 depends on a peculiar optical illusion not yet understood {Tetraspora lubricd). The 

 end which carries the cilia usually goes forwards, but it may also go backwards, and 

 it then rotates in the contrary direction (Ulolhrix speciosd). This reversion occurs 

 when the swarmers rebound : they then rotate for a time at one spot, stand still, 

 and go back (without turning round the body). The reversal of the rotation only 

 takfes place, however, in so far that the end bearing the cilia is always regarded as 

 the anterior one; if, on the contrary, the end -which is in front during the pro- 

 gressive movement (even when reversed) is termed anterior, then the direction of 

 rotation is always the same, the backward movement always continues for a short 

 time only, and is soon exchanged again for the usual one. 



The progressive and rotating movements stand also with respect to their velocity 

 in a relation not exactly determined, and both are, according to Naegeh, apparently 

 due to the same cause. As a rule the one increases or decreases with the other ; but 

 if a swarm-cell strikes anything it remains at the spot, but continues to rotate, and 

 occasionally one may be seen to move forwards without revolving. Tn the absence of 

 all obstructions, however, both movements appear to be always combined. On the 

 other hand, cells are also seen which, with an equal number of revolutions in the unit 

 of time, swim forwards with unequal rapidity, or with equally rapid progress revolve 

 with unequal rapidity. — In these matters- there are evidently individual differences 

 (depending upon the organisation). Swarm-cells occurring in the field of view at the 

 same time, and thus exposed to the same external conditions, move with unequal 

 velocities : the cells of Tetraspora lubrica, for instance, traverse at I4°C. a space of 

 one-fifth of a mm. in I-2-2-4 seconds, and revolve, striking against the upper or lower 

 glass plate, once in o-3-i-8 seconds. — Heat also accelerates these movements of 

 protoplasmic structures. According to Unger, the swarm-spores of Vaucheria 

 traversed a distance of one inch in 63-65 seconds. 



The fact must not be overlooked that swarm-spores and spermatozoids com- 



