STRUCTURE AND FUNCTION IN MAMMALIAN EGGS «S7 



the zona pellucida, egg and follicle cells become separated, but the 

 microvilli continue to project up to about half-way through the 

 membrane, and many of the follicle-cell processes, passing com- 

 pletely through, retain contact with the vitelline surface (Fig. 47). 

 The microvilli diminish and disappear shortly before ovulation 

 (Yamada, Muta, Motomura and Koga, 1957; Moricard, 1958; 

 Chiquoine, 1959, i960; Sotelo and Porter, 1959; Anderson and 

 Beams, i960; Odor, i960). 



The vitelline membrane must be intimately involved in the 

 attachment of the spermatozoon to the vitelline surface, and in at 

 least the initial phases of sperm engulfment. Observations in rat 

 eggs show that the sperm head usually comes to lie flat upon the 

 vitelline surface and to remain thus for an appreciable time before 

 it is engulfed (see Austin and Braden, 1956); a similar relationship 

 has also been reported in the rabbit (Dauzier and Thibault, 1956). 

 Particles taken into phagocytic cells apparently continue to be 

 surrounded by plasma membrane, and thus in a sense remain 'out- 

 side' the cell. Sperm penetration has points of resemblance with 

 phagocytosis (Loeb, 19 17) and spermatozoa seem prone to engulf- 

 ment by various cells: they are known to be taken up readily by 

 macrophages (Hoehne, 1914; Hoehne and Behnc, 19 14) and poly- 

 morphonuclear leucocytes (Yochem, 1929; Merton, 1939; Austin, 

 1957c), and apparently even by epithelial cells (Austin and Bishop, 

 1959b; Austin, 1959a, 1960a). In addition, the appearances presented 

 by the ultra-thin section of the hamster egg shown in Fig. 70 are 

 consistent with the idea of phagocytosis — the sperm tail is apparently 

 contained within a vesicle in much the same way as a phagocytosed 

 particle, and the vesicle is presumably limited by an invaginated 

 portion of the vitelline membrane. Nevertheless, recent observa- 

 tions of Szollosi and Ris (1961), based on electron micrographs of 

 rat spermatozoa in the act of entering the vitellus, make it clear that 

 the mechanism involved is essentially different from phagocytosis 

 (see Frontispiece). These authors postulate that, when the fertilizing 

 spermatozoon comes into contact with the vitellus, the cell mem- 

 branes of both the spermatozoon and the egg rupture in the area of 

 contact and unite with each other. The sperm cell membrane thus 

 becomes continuous with the vitelline membrane and is left behind 

 on the surface of the vitellus as the spermatozoon passes into the 

 cytoplasm. Membrane fusion is held to entail the force responsible 

 for the movement of the spermatozoon into the vitellus. Similar 



