STRUCTURE AND FUNCTION IN MAMMAU \N EGGS 57 



two occasions: at the time of expulsion of the first polar body and 

 shortly after the entry of the spermatozoon. In the former instance, 

 the change in volume is responsible for transforming the peri- 

 vitelline space from a potential state to a real one. The contraction 



Fig. 47 

 Relations between follicle cells (stippled), 

 zona pellucida (horizontal lines) and vitellus 

 (black) in the late ovarian oocyte, as revealed 

 by published accounts based on electron 

 microscopy. (Semi-diagrammatic.) 



following sperm entry is generally taken as a feature of activation, 

 and, indeed, it is also clickable by the various stimuli that are known 

 to be capable of initiating parthenogenetic development (p. 38). 

 Dauzier and Thibault (1956) maintain that contraction can be 

 induced in vitro by the mere presence of spermatozoa in the medium. 

 The decrease in volume after sperm entry has been observed in the 

 rabbit (Gregory, 1930; Pincus and Enzmann, 1932; Thibault, 1947- 

 1949), mouse (Sobotta, 1895), guinea-pig (Lams and Doormc, 

 1908), dog, cat and bat (Van der Stricht, 1923), rat (Gilchrist and 

 Pincus, 1932; Pincus and Enzmann, 1934; Pincus, 1936a; Austin 

 and Braden, 1954b), cow (Hamilton and Laing, 1946), hamster 

 (Austin, i956d) and pig (Pitkjanen and Sheglov, 1958). It has been 

 estimated to represent some 13 to 17 per cent of the vitelline volume 

 in the rat egg and about 9 per cent in the hamster egg, but was too 

 small for accurate assessment in the diminutive egg of the field vole 

 (Austin, 1957b). Krassovskaja (1935b) reports that the rabbit egg 

 increases in volume after the formation of the pronuclei and up to 

 the stage of the formation of the cleavage spindle. 



