50 



THE MAMMALIAN EGG 



eir 



irregularly shaped masses, often with a complex structure; th 

 nature is conjectural. 



After a time, changes occur in the 2-cell nucleus that presage the 

 next mitosis. The nucleus decreases in volume, the nucleoli diminish 



Fig. 33 

 Nuclei from rat 2-cell eggs, showing nucleolar inclusions. X 1,200. 



in size and number and disappear, and the chromosomes condense — 

 the course of events is similar to the first-cleavage prophase changes 

 of the pronuclei. Mitosis then advances through metaphase and 

 anaphase to telophase, the cytoplasm undergoes division, and nuclei 

 are reconstituted. Nuclear and nucleolar volumes are approximately 

 halved at each stage, and the number of nucleoli is reduced (Hert- 

 wig, 1939; Austin and Braden, 1953c) (Fig. 12). The overall size of 

 chromosome groups and the chromosomes themselves become 

 progressively smaller. By contrast, the nucleolus-associated material, 

 just discernible at the 2-cell stage, becomes increasingly prominent, 

 and, by the 16-cell stage in the rat, the perinucleolar elevations are 

 so large that they often conceal the nucleoli (Fig. 34a to d). Ultra- 

 violet microscopy at a wavelength of 2,600 A shows that the 

 material composing the elevations contains a high concentration of 

 nucleic acid, whereas the nuclear sap and the nucleoli have very 

 much less (Austin, 1953; Austin and Braden, 1953c) (Fig. 34c to j). 

 Observations by fluorescence microscopy, with acridine orange as 

 vital fluorochrome, reveal a similar distribution and indicate that 

 the nucleic acid in question is dna (Figs. 35 and 36). Histological 

 studies with Fculgen's reagent applied to fixed material provide 

 confirmation (Alfert, 1950; Braden and Austin, 1953), and it is 



