30 



THE MAMMALIAN EGG 



Fig. 24 

 The egg of the field vole Microtus agrestis. X 900. (From Austin, 1957b.) 



1959), the two pronuclei often do not differ appreciably in size. 

 The pronuclei of rodent eggs in general seem to be characterized 

 by being relatively big (nucleocytoplasmic ratio about i : 30) and 

 having relatively large nucleoli ; by contrast, the rabbit egg shows a 

 ratio of about 1 : 90 and nucleolar volume constitutes only about 

 1 per cent of the pronuclear volume. 



Properties of Pronuclei 



During its formation, and before nucleoli become visible, the 

 incipient pronucleus appears by all tests as a dense accumulation 

 of dna (Fig. 25), but the concentration soon diminishes as the 

 pronucleus grows (Alfert, 1950; Braden and Austin, 1953; Ludwig, 

 1953, 1954; Austin and Bishop, 1959a; Austin and Amoroso, 1959). 

 It is reasonable to suppose that the diminution is owing to a dilution 

 effect attributable to the great increase in volume that occurs during 



