48 THE MAMMALIAN EGG 



aster. There appeared to be a mechanism in the egg which, under 

 normal circumstances, could be held responsible for co-ordinating 

 the development of the pronuclei. Correlation of a similar kind has 

 been observed also in several phases of mammalian fertilization. In 

 the rat, the first nucleoli make their appearance at about the same 

 time in both pronuclei; the pronuclei reach their maximum size 

 together and, later, start simultaneously upon the process of syn- 

 gamy. Polyspermic (dispermic) rat eggs, too, provide evidence of 

 co-ordination in the striking similarity of form exhibited by the 

 two male pronuclei at the various stages of pronuclear development 

 (Austin, 1951c; Austin and Braden, 1953b, 1954b). 



Attempts to disturb the synchrony of development of pronuclei, 

 by treatment with colchicine, cold shock or heat shock, yielded only 

 transient effect, the induced disturbance soon becoming corrected 

 (Austin and Braden, 1954b). In mouse eggs penetrated by X-irradi- 

 ated spermatozoa, the pronuclei often developed well but failed to 

 enter upon syngamy; it was surmised that irradiation had impaired 

 the male pronucleus, rendering it incapable of proceeding further, 

 and that the female pronucleus was unable to go forward alone 

 (Bruce and Austin, 1956). These observations add support for the 

 idea that, in eggs as in tissue cells, the cytoplasm exerts a controlling 

 influence over nuclear function, an idea for which a solid bio- 

 chemical foundation has already been laid through work on tissue 

 cells (see Brachet, 1957). 



Nucleocytoplasmic relations in the synthesis of dna are discussed 

 in the next section. 



Cleavage Nuclei 



Fertilization may be said to end with the condensation of the 

 chromosomes in the male and female pronuclei and the coming 

 together of the two chromosome groups to form a single group. 

 These events can also be regarded as constituting the prophase of 

 the first cleavage mitosis, for the chromosomes proceed immediately 

 thereafter to become arranged as the metaphase plate of the first 

 cleavage spindle. There is now evidently a pause, since eggs re- 

 covered from rats at about the time of the first cleavage are more 

 often found in metaphase than in stages just preceding or succeeding. 

 The mitosis passes to telophase, cytoplasmic division occurs, and 

 interphase nuclei arc reconstituted from the chromosome groups 

 (Fig. 32). The mode of formation of the nuclei resembles that of 



