STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 27 



largest number of nucleoli recorded in one series of observations on 

 the rat male pronucleus were 17 and 36, respectively (Austin, 1952a) ; 

 the second figure is well in excess of the number of chromosomes 

 that would be present (N = 21). If pronuclear nucleoli are formed 



J 



k l 



Fig. 18 

 Pronuclear development and syngamy in the rat egg. (a)-(d) Later phase in the growth 

 of the male pronucleus, (e) Male pronucleus (below) and female pronucleus (above) at the 

 start of syngamy. (/)-(/) Condensation and conjugation of chromosome groups. (Drawn 

 from photographs; the changes from (e) to(g) and from (h) to (/) were observed as continuous 

 processes that occurred in vitro in separate eggs.) 



at specific nucleolus-organizing loci on chromosomes, as is the case 

 in tissue cells, it must be surmised either that pronuclei possess 

 numerous nucleolus organizers (more than one per chromosome), 



