KARYOKINESIS 



257 



The Extrusion of Polar Bodies. 



of Polar Bodies may be described as 



Extrusion 

 follows : — 



Immediately before actual fertilisation — though the 

 spermatozoon may have entered the ovum — the germinal 

 vesicle, or nucleus of the ovum, begins to lose its definite 

 form and to move towards the periphery of the ovum. 

 Gradually the germinal spot and vesicle disappear, and 

 two " star-like " arrangements united to form the poles of 



'^ 



Fig. 93. — Formation of Polar Bodies In Asterias glaeialis. 

 I. The nuclear spindle approaches the surface. II. One half 

 of the spindle is projected in an elevation. III. The elevation 

 is separated as the first polar body, Pi, and the inner half of 

 the original spindle becomes again a complete spindle. IV. 

 This spindle is protruded to half its extent. V. The protrusion 

 becomes the second polar body, p2. VI. The deep part of 

 the spindle becomes the nucleus, n, of the ovum. (After 

 Hertwig.) 



a spindle make their appearance in connection with the 

 germinal vesicle, and one pole turns round till it comes 

 in contact with the surface of the yelk and is gradually 

 separated by constriction from the other half, constituting 

 what is called the " first polar body." 



" The remaining half of the spindle is again converted 

 into a complete spindle, and a second time one half is pro- 

 jected from the surface of the yelk and separated to produce 



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