CH. II] POLAR BODIES AND FERTILIZATION 25 



surface of the egg, or if a part enter the egg it takes no share 

 in the subsequent changes. The middle piece of the sperma- 

 tozoon is now known to contain a body that plays a most con- 

 spicuous part in many animals in the division or cleavage of 

 the egg. The middle piece enters with the head of the sper- 

 matozoon. It contains the centrosome, which divides, and 

 around each centre an elaborate system of rays develops. The 

 two centrosomes migrate to opposite sides of the segmen- 

 tation-nucleus, and between the two appears the spindle of 

 the first cleavage. In the frog the history of the middle 

 piece and centrosome, and the origin of the segmentation- 

 spindle have not yet been worked out. 



