STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 



17 



histological preparations, the nucleus is more or less spherical in 

 shape and contains one or a very few nucleoli and either irregular 

 chromatin masses or bodies recognizable as incompletely condensed 

 chromosomes. 



(e) 



Fig. 12 

 Relative nuclear size (rat) in (a) early primary oocyte, (b) late primary 

 oocyte, (c) egg in late stage of fertilization, (d) 2-cell egg, (e) 4-cell egg, 

 (/) 8-cell egg, (g) follicle cell, and (h) spermatozoon. 



The oocyte undergoes considerable enlargement before it is ready 

 for ovulation, the increase in volume in the rat being of the order 

 of ninety fold. The volume of the nucleus increases proportionately; 

 in the living rat oocyte, it reaches about 18,000 /x 3 , which is more 

 than the entire size of most tissue cells (Fig. 12). When the nucleus 

 is examined by phase-contrast microscopy, it is seen to be spherical 

 in shape and to contain generally a single large, excentrically placed, 

 highly refractile nucleolus and some small granular masses of 

 irregular form. Within the nucleolus, there is often a spherical 

 vacuole which may be quite large and appears to contain nucleo- 

 plasm. Examined by ultra-violet and fluorescence microscopy 

 (pp. 107-108), it is evident that material containing a high concen- 

 tration of dna exists as a thick shell about the nucleolus and in the 

 irregular granular structures nearby (Austin and Braden, 1953c; 

 Austin and Bishop, 1959a) (Figs. 13 and 15). The nucleolus itself 

 appears to contain some rna but the nuclear sap is virtually devoid 



