464 



MOUSE EMBRYO 



Vaginal plug of female mouse the morning after successful 

 mating. This plug arises from secretions from the male, 

 and is certain sign of copulation. Usually it means suc- 

 cessful fertilization as well, but there are species differ- 

 ences so that in some strains this is not a guarantee of fer- 

 tilization. This plug generally disappears within a few 

 hours after mating. 



^^?i:^5*,^iS^ 



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w 





Entire litter of mice liberated from but still attached to 

 the uterus by umbilical cords, at 17. S days gestation. 



(rf) 

 The figures illustrate four stages in the penetration of a 

 rodent egg by a spermatozoon, (a) The spermatozoon head 

 has just passed through the zona pellucida and made contact 

 with the vitellus. (b) The spermatozoon head is lying flat 

 upon the vitelline surface to which it is now attached, 

 (c) The whole of the spermatozoon mid-piece has entered 

 the egg and the head has passed through the surface of the 

 vitellus. The spermatozoon head shows an early phase in 

 its transformation to a male pronucleus- -the posterior end 

 of the head is becoming indistinguishable from the egg 

 cytoplasm, (d) The head and mid-piece of the spermato- 

 zoon have now entered the vitellus. Transformation of the 

 head has proceeded to the stage immediately before the 

 appearance of nucleoli. The cytoplasmic elevation over 

 the spermatozoon head, just evident in (c) has now become 

 much larger. From Austin C. R. , A. W. H. Braden, 1956, 

 Jour. Exp. Biol. 33:360. 



