60 ANIMAL PHYSIOLOGY. 
etc.), then, are simply expelled ; they take no part in the devel- 
opment of the ovum; and their extrusion is to be regarded as a 
* preparation for the progress of the cell, whether this event fol- 
lows or precedes the entrance of the male cell into the ovum. 
It is worthy of note that the ovum may become ameeboid in the 
region from which the polar globules are expelled, 
The remainder of the nucleus (female pronucleus) now 
passes inward to undergo further changes of undoubted im- 
portance, possibly those by virtue of which all the subsequent 
evolution of the ovum is determined. This brings us to the 
consideration of another cell destined to play a brief but im- 
portant réle on the biological stage. 
- THE Mae CELL (Spermatozo6n),. 
This cell, almost without exception, consists of a nucleus 
(head) and vibratile cilium. However, as indicating that the 
a 
Fic. 64.—Spermatozoa (after Haddon). Not drawn to scale. 1, sponge; 2, hydroid ; 3, nema- 
tode; 4. cray-fish ; 5, snail; 6, electric ray ; 7, salamander ; B horse ; 9, man. ‘In many 
spermatozoa, as in Nos. 7 and 9, an extremely delicate vibratile band is present, 
