32 THE MAMMALIAN EGG 



membrane on the side nearest the male pronucleus. Such a distribu- 

 tion of chromatin in pig pronuclei has been recorded also by Han- 

 cock (1961). 



The highly refractile nucleoli are striking features of the pro- 

 nuclei. Centrifugation of pronuclear eggs causes the nucleoli to 

 coalesce and makes it clear, too, that they are appreciably denser 

 than most other components of the egg (Dalcq, 195 1, 1952). If a 

 living egg is ruptured whilst under examination, the nucleoli are 

 often set free into the surrounding medium and can then be seen to 

 behave rather in the manner of oil droplets (again suggesting that 

 they have distinct surface tension, c.f. p. 25). Constrained by 

 movement of the medium to pass through a narrow space between 

 cell fragments, the nucleoli readily deform and break up into 

 smaller bodies which immediately resume the spherical shape. 

 Quite often, a nucleolus is found to contain a spherical inclusion 

 (Fig. 24); these inclusions vary greatly in diameter, as do those 

 of the nucleoli in cleavage nuclei (see p. 49 and Fig. 33). The 

 material within the inclusion resembles nucleoplasm in appearance; 

 occasionally, nucleoli with large inclusions are seen to 'break', 

 releasing the contents of the inclusion, which mixes freely with the 

 nucleoplasm. In the field vole, pronuclear nucleoli may show the 

 presence of a small body within an inclusion, the arrangement 

 suggesting a 'bull's-eye' in appearance. The small inner body seems 

 likely to be a fragment of nucleolar material. Throughout pro- 

 nuclear life, the nucleoli appear to be free of nucleic acid: they show 

 negligible ultra-violet absorption (Austin and Braden, 1953c), no 

 detectable fluorescence (Austin and Bishop, 1959a; Austin and 

 Amoroso, 1959) and are acidophilic and not basophilic when tested 

 under controlled ionic conditions (Braden and Austin, 1953). It 

 seems likely, therefore, that they consist largely of basic protein. 

 They stain with pyronine (Odor and Blandau, 1951b), but this reac- 

 tion is of uncertain significance. They stain orthochromatically 

 with toluidine blue and often contain metachromatic inclusions 

 (Izquierdo, 1955) which also give a positive reaction with the 

 periodic acid-Schiff test (Dalcq, 1955a) and which can be regarded 

 as consisting probably of mucopolysaccharides. Nucleoli have been 

 reported to contain phospholipid (Dalcq, 1954a, b) and alkaline 

 phosphatase (Mulnard, 1955). 



Early investigators, using the older histological methods, often 

 described pronuclear nucleoli as being of two or three different 



