26 THE MAMMALIAN EGG 



of 'wetting' the nuclear membrane. This relationship between 

 nucleolus and nuclear membrane has also been noted by Sotelo and 

 Porter (1959) in electron-microscope studies of rat eggs. They 

 maintain that both layers of the nuclear membrane are continued 

 around the indenting part of the nucleolus which is therefore fully 

 within the nucleus and not projecting into the cytoplasm. When 

 the pronuclei have reached their maximum size, they move together 

 and come into intimate contact with each other in the centre of the 

 egg. After a pause, syngamy is initiated: the pronuclei begin to 

 decrease in size and some of the nucleoli undergo coalescence. 

 Reduction in volume then affects both pronuclei and nucleoli and 

 continues until the pronuclei reach about half their maximum size. 

 The nuclear membrane now disappears, as the last of the nucleoli 

 fade out, and the nuclear sap assumes the consistency of a gel, 

 within which the condensing chromosomes become visible. The 

 two chromosome groups move together making a single group 

 which resolves itself into the metaphase plate of the first cleavage 

 spindle. The gathering together and possible intermingling of the 

 chromosome groups deriving from male and female pronuclei is 

 the consummation of the fertilization process (Figs. 18 and 20). It 

 is characteristic of mammals that intermingling does not occur until 

 this point, the final phase of syngamy ; the formation of a zygote 

 nucleus by union of male and female pronuclei, which takes place 

 to varying degrees in invertebrates (see Wilson, 1928), is not known 

 in mammals, with the possible exception of the monotremes. 

 According to Flynn and Hill (1939), when the pronuclei of Echidna 

 become apposed the nuclear membranes over the area of contact 

 disappear and a single cleavage nucleus is formed. 



In the rat, the volumes of the pronuclei and the numbers of 

 nucleoli reach their maxima in about half the pronuclear life-span, 

 and the levels are maintained until the start of syngamy. Nucleolar 

 volume increases more rapidly so that the maximum is reached in 

 about a quarter of the pronuclear life-span; in the early male 

 pronucleus, the increase in nucleolar volume initially outstrips that 

 of nuclear volume so that coalescence and reduction in number of 

 nucleoli occur, but later the enlarging pronucleus is able to accom- 

 modate extra nucleoli. Pronuclear growth involves an enormous 

 increase in volume : the nucleus of the rat sperm has a volume of 

 the order of 10 /x 3 and the male pronucleus at full development 

 about 5,500 jit 3 , an increase of 550 times (Fig. 12). The mean and 



