STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 49 



the female pronucleus after the second meiotic division, numerous 

 minute nucleoli appearing in the midst of the fading chromosome 

 group, while an encircling nuclear membrane becomes visible. 

 Some coalescence of nucleoli is associated with the subsequent 



Fig. 32 

 Stages of cleavage in the rat egg. (From Austin, 1959c.) 



growth of the nuclei. When fully grown, the nuclei of living 2-cell 

 rat eggs are similar in general structure to pronuclei, except that 

 fewer nucleoli develop and small elevations of material can often be 

 seen on the surface of some of the nucleoli. Nucleolar inclusions, 

 too, are occasionally met with, ranging from small spherical bodies 

 with a diameter a fraction of that of the nucleolus to others so large 

 that the nucleolar substance is reduced to a mere shell (Fig. 33). The 

 inclusions seen in 2-cell rat eggs are evidently composed of fluid 

 like the nuclear sap, for sometimes a nucleolus with a large inclusion 

 may be observed to 'break', releasing the contents which mingles 

 immediately with the nuclear sap. The nucleolar substance then 

 rapidly assumes a spherical form, now much smaller in diameter 

 than before. The nuclei of living 2-ccll rat eggs examined by phase- 

 contrast microscopy were often found to contain other structures 

 than nucleoli and their attached material. These were small, 



