44 



THE MAMMALIAN EGG 



presence initially of the extra male pronucleus and later of the extra 

 chromosome group. The three chromosome groups that eventually 

 become evident are similar in appearance and they move together 



© 



<§ 



® 



® 



Fig. 30 



Pronuclei in rat eggs, (a), (b), (c) and (e) Stages in the development of polyandry arising 

 from dispermy, showing the close similarity throughout between the two male pronuclei. 

 (d), (e) and(/) Pronuclei at full development after monospermic, dispermic and trispermic 

 penetration, respectively. (Drawn from photographs.) 



to form a single gathering in the centre of the egg. Almost in- 

 variably, a normal-looking bipolar spindle was found to have 

 formed (Fig. 31), despite the triploid number of chromosomes, and 

 the first cleavage division seemed to go through in the usual way. 

 Polyandric early embryos could be recognized by the possession of 

 two sperm tails in the cytoplasm, and such embryos, normal in 



