650 



REPRODUCTIVE ORGANS. 



for but a few weeks, that which occurs in connection with 

 nancy remains for a much longer period, even until after c 



Mid- 



FIG. 434. Portions of ova of Asterias gladalis, showing changes affecting the 

 germinal vesicle at the beginning of maturation : a, germinal vesicle ; 6, germinal 

 spot, composed of nuclein and paranuclein (c) ; d, nuclear spindle in process of 

 formation (Hertwig). 



FIG. 435. Formation of polar bodies in ova of Asterias gladalis : ps, polar 

 spindle; pb', first polar body ; pb", second polar body ; n, nucleus returning to con- 

 dition of rest (Hertwig). 



FIG. 436. A, mature ovum of echinus : n, female pronucleus ; B, immature ovarian 

 ovum of echinus (Hertwig). 



birth. The manner of formation is, however, the same in both, 

 only under the stimulus of pregnancy the corpus luteum which 



