190 ORGANISED FLUIDS. 



Sometimes, indeed, when the semen is thick and tenacious, 

 the movements of the contained spermatozoa are but feeble, 

 the density of the liquor seminis presenting too great an 

 impediment to the free motion of these minute creatures. 



Often, however, in such cases, when the fluid has been 

 diluted with water or with some other liquid, as serum 

 or milk, of less density than itself, the spermatozoa, being 

 now set at liberty, will frequently be seen to resume their 

 locomotive powers, and to move about with the greatest 

 activity. All the spermatozoa, however, contained in a drop 

 of semen which has undergone dilution will not start into 

 motion at once ; many of them will remain for a time per- 

 fectly motionless, and then suddenly, and, as it were, by an 

 act of volition, begin to move themselves in all directions. 



Mode of Progression. 



The motions of the spermatozoa are effected principally by 

 means of the tail, which is moved alternately from side to 

 side, and during progression the head is always in advance. 



The strength of the spermatozoa is considerable, it en- 

 abling them, when they are immersed in either blood or 

 milk, to cast aside, with the greatest ease, the globules which 

 may present themselves to impede their progress. 



The spiral spermatozoa of the Passeres advance by a move- 

 ment of rotation of the body, the tail remaining extended 

 and motionless, acting rather as a rudder than as an organ 

 of locomotion. The spermatozoa of the other orders of birds, 

 and which consist of a cylindrical body to which a short and 

 attenuated tail is attached, " scull themselves forward with 

 their tails, either striking them slowly and with wide sinu- 

 osities, or more quickly and shortly, as when a whip is 

 shaken ; they thus advance in circles with a quivering mo- 

 tion, holding the body extended in a straight line, although 

 they also now and then bend this in various directions from 

 side to side."* 



* Wagner's Elements, p. 18. 



