is 



THE MAMMALIAN EGG 



of nucleic acids. Histological studies yield similar results; tests with 

 ribonuclease show specifically the presence of rna in the nucleoli 

 (Vincent and Dornfeld, 1948). The total amount of dna in the 

 oocyte nucleus throughout oocyte growth has been shown to be 



Fig. 13 

 Rat oocyte nucleus photographed by (a) phase-contrast 

 microscopy, and (/>) ultra-violet microscopy (at 2,600 A). X 

 800. (From Austin and Braden, 1953c.) 



constant at the tetraploid level, the concentration falling during 

 growth, presumably through dilution with increasing nuclear 

 volume (Vincent and Dornfeld, 1948; Alfert, 1950; Van de 

 Kerckhove, 1959). Experiments with glycine-2- 14 C show that the 

 tracer accumulates particularly in the nucleolus and its shell, in 

 accordance with current ideas on protein and nucleic-acid synthesis 

 (Edwards and Sirlin, 1958). The material composing nucleoli 

 appears to have a higher specific gravity than the other constituents 

 of nucleus and cytoplasm (Dalcq and Van Egmond, 1953). 



By electron microscopy, the oocyte nucleus in the mouse and 

 rat was found to be occupied chiefly by a finely granular mass 

 representing the nucleoplasm and limited by a double membrane, 

 in which the characteristic pores could be discerned (Yamada, 



