STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 47 



been found in rats after induced hyperthermia, but their nuclear 

 state was too irregular to justify their description as truly quinque- 

 nuclear and sexinuclear eggs (Austin, 1955, 1956b). 



NUCLEOCYTOPLASMIC RELATIONS IN FERTILIZATION 



Certain aspects of nuclear development in eggs testify to the 

 closeness of the nucleocytoplasmic interdependence recognized as 

 a feature of cells in general. It is a common observation that the 

 chromosome groups emitted within polar bodies often do not give 

 rise to resting nuclei, and, on those rare occasions when the sperm 

 head becomes lodged in a polar body, or extruded from the vitellus 

 in a small mass of cytoplasm, it too fails to give rise to such a nucleus. 

 Presumably, the organelles that normally participate in nucleus 

 formation are often lacking from polar bodies; in addition, polar 

 bodies would probably be deficient in the necessary substrate. That 

 the availability of substrate material is a limiting factor in pronuclear 

 growth is strongly suggested by the subnormal size exhibited by 

 pronuclei in polyandric and polygynic eggs. This limitation in 

 growth stands in strong contrast to the supernormal size achieved 

 by female pronuclei in rudimentary parthenogenesis or gynogenesis. 

 Substrate availability is, however, evidently not the only condition 

 that determines the ultimate size of pronuclei. The volumes of 

 single nuclei developing in eggs were found to be less than the 

 combined volumes of normal male and female pronuclei, so it is 

 inferred that there must be yet another restricting influence, possibly 

 inherent in the nuclei themselves (Austin, 1952a; Austin and Braden, 

 1955). Such an influence, predominating in the female pronuclei of 

 eggs such as those of the rat and mouse, could underlie the large 

 difference in relative size of male and female pronuclei. On the 

 other hand, this pronuclear disparity could be ascribed equally well 

 to a greater affinity of the male pronucleus for cytoplasmic substrate. 

 Suppression of pronuclear development, apparently by influences 

 arising in or mediated by the cytoplasm, has been described in 

 urodele eggs : in polyspermic fertilization, the supernumerary male 

 pronuclei regress when syngamy is effected between the female 

 pronucleus and the successful male pronucleus (Fankhauser, 1948). 

 Evidence of a different nature was provided some years ago by 

 Brachet (1922) who noted that the development of the male pro- 

 nuclei and associated asters in polyspermic sea-urchin eggs proceeded 

 exactly synchronously with that of the female pronucleus and its 



