THE CELL 15 



for the reproduction of the species. The spermatozoon 

 in its gross anatomy has the semblance of a very minute 

 tadpole swimming freely about in liquids. It has a 

 distinct head piece and a long slender tail piece or cilium. 

 (See Fig. 4.) When closely examined, the sperm-cell 

 exhibits all the essential characteristics of a typical cell, 

 with a nucleus, cytoplasm and a cell-wall. This cell is 

 so small that in some forms it is but iooVoo of the size 

 of the egg-cell. The nucleus occupies almost the entire 

 space available in the head piece, being surrounded by 

 a very thin layer of cytoplasm between it and the cell- 

 wall. The tail of the sperm-cell is joined to the head 

 piece by the middle piece. It is of cytoplasmic origin 

 and possesses the power of motion. This ability to propel 

 itself forward in liquid media seems to be in a way an 

 insurance that the sperm-cell will ultimately approach 

 the egg-cell for the purpose of fertilization. 



The power of motion is retained by the spermatozoon 

 for a considerable time under favorable conditions of 

 warmth and moisture. After the sperm enters the egg, 

 it loses the power of motion and the tail piece is absorbed. 



The essential portions of the sperm which are concerned 

 in the process of fertilization are the nucleus and the 

 middle piece. Other structures which are accessory to 

 these are the apex by which the sperm attaches itself 

 to the ovum and the tail whose functions have already 

 been described. 



