Polar Bodies. 



123 



spindle are two rounded bodies whose origin and fate is at 

 present a mystery, but which have been called the centrosomes ; 

 from the centrosomes radiate a number of lines of granules of 

 the egg protoplasm into the surrounding mass. 



The entire nucleus advances towards the periphery, and half 

 of the chromosomes, together with a portion of the spindle, sepa- 

 rates itself from the egg, and remains at the surface as the first 

 polar body. A second polar body is then protruded, and the ovum 



f.pn.- 



Fig. 61.— Stages in the formation of polar bodies in the ovum of a Starfish. (After Hertwig.) 



ff.v., germinal vesicle transformed into a spindle-shaped system of fibres ; p', the first 

 polar body becoming extruded ; ^,^, both polar bodies fully extruded ; f.p^., female 

 pronucleus, or residue of the germmal vesicle. 



after this is ready for fertilisation. The extruded polar bodies 

 play no further part ; they remain for some time, and ultimately 

 disappear. Their nature will be discussed later. 



History of the Spermatozoa. 



The spermatozoa of animals are very different from their ova. 

 While the ova are always relatively large, sometimes enormous, 

 lethargic, showing at most amoeboid movements, the spermatozoa 

 are minute, and nearly always . actively motile. Even where 

 they are immobile, as is the case with the Crustacea, their form 

 is different from that of the ova, and they are greatly smaller. 



