STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 31 



pronuclcar growth. In living eggs recovered during the early part 

 of pronuclear growth, when the dna can still be detected (by 

 ultra-violet absorption and induced fluorescence — Fig. 26), it seems 

 to be distributed evenly in the nucleoplasm, thus presenting a clear 

 difference from the nuclei of oocytes, cleaving eggs and tissue cells 

 in which most of the dna appears in aggregate form. In histo- 

 logical sections of early pronuclear eggs stained with Feulgen's 

 reagent or methyl green, the dna is found especially around the 

 nucleoli and lining the nuclear membrane — in view of the appear- 

 ance in living eggs, this distribution seems likely to have been 

 produced by fixation. When the pronuclei reach their full size, 

 dna cannot be detected with certainty by ultra-violet absorption 

 or by Feulgen or methyl-green staining, but there is still visible in 

 the nuclear sap a faint green fluorescence following treatment with 

 acridine orange. Later, as the time of syngamy approaches, the 

 green fluorescence is found to have become distinctly stronger and 

 dna can once more be demonstrated by histological methods. 

 Measurements of total dna content show that the amount doubles 

 during the pronuclear life-span, the complement in individual 

 pronuclei ranging from the haploid quantity to the diploid (Alfert, 

 1950). In mice injected a few hours before ovulation with 

 adenine-8- l4 C, the earliest synthesis of dna by the pronuclei, as 

 detected by labelling, was evident about 13 hr after ovulation 

 (or about 11 hr after the estimated time of sperm penetration) 

 (Sirlin and Edwards, 1959). Later, chromosome condensation in 

 the prophase of the first cleavage division is apparent in the local- 

 ization of dna near the nuclear membrane in each pronucleus, 

 particularly in the region where the pronuclei are in contact. 

 Finally, the condensed chromosomes gather in the single large 

 tetraploid group from which the metaphase plate of the cleavage 

 spindle develops. 



In histological preparations, differences have been observed in 

 the staining reactions of male and female pronuclei: in the pig 

 (Pitkjanen, 1955; Thibault, 1959), rabbit (Dauzier and Thibault, 

 1956), hamster (Hamilton and Samuel, 1956). In the hamster, the 

 larger paler-staining female pronucleus is said to be readily distin- 

 guished from the smaller darker-staining male pronucleus. Late- 

 phase female pronuclei in the rabbit and pig are described as being 

 asymmetrical, owing to the gathering of chromatin near the nuclear 



