90 THE MAMMALIAN EGG 



contains ascorbic acid. Koneckny (1959) reported that the nieta- 

 chromasia exhibited by the external and especially by the most 

 internal layers of the zona pellucida of cat follicular oocytes is 

 removed by treatment with hyaluronidase ; from this, it was inferred 

 that hyaluronic acid is a normal component of the zona. Strong 

 staining of the membrane with Sudan B was interpreted to indicate 

 the presence of lipoprotein. Solution of the zona pellucida is 

 obtained with acid media: pH 4-5 to 5 for the rat zona pellucida, 

 pH 3 for the rabbit, pH 2-8 for the hamster, pH 2-4 for the field 

 vole (Hall, 1935; Harter, 1948; Braden, 1952; Austin, i956d, 1957b). 

 The zona pellucida is digested by some enzymes and not by others, 

 distinct species differences being displayed (Table 4). It appears to 

 be morphologically unaffected by hyaluronidase. The mouse, rat 

 and rabbit zona pellucida is digested by trypsin more readily before 

 sperm penetration than after (Smithberg, 1953; Chang and Hunt, 

 1956), a change that presumably reflects the occurrence of a zona 

 reaction. The zona pellucida of rabbit, rat and hamster eggs is 

 permeable to substances of a molecular weight of 1,200 or less, but 

 not to those of m.w. 16,000 (Austin and Lovelock, 1958). This 

 means that the vitellus can be considered directly accessible to all 

 the known essential food components, including vitamins, to the 

 great majority of pharmacologically active compounds, and to all 

 natural steroid hormones. It would be inaccessible to most enzymes, 

 antigens, antibodies, protein hormones, and substances of the nature 

 of the invertebrate fertilizins and antifertilizins. 



Passage of spermatozoa through the rodent zona pellucida is a 

 very rapid process, judging from the infrequency with which eggs 

 are recovered with spermatozoa in the act of penetrating this 

 membrane. It has been remarked by some of the investigators who 

 have recorded mouse and rat eggs in this condition that the sper- 

 matozoa appeared to be in the act of passing obliquely through the 

 zona pellucida (Sobotta, 1895; Sobotta and Burckhard, 1910); more 

 recent observations certainly support this idea, for not only have 

 sperm heads regularly been found to lie obliquely in the thickness 

 of the zona pellucida in hamster and guinea-pig eggs, but the slits 

 left in the zona by penetrating spermatozoa, as observed in guinea- 

 pig and Libyan-jird eggs, were found to follow a curved, oblique 

 path (Austin and Bishop, 1958c). No adequate reason has yet been 

 advanced to account for this direction of penetration. 



