STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 101 



concentric layers between which debris and occasional cells, includ- 

 ing spermatozoa, may be trapped. Not only eggs but other objects 

 also, such as fragments of sloughed epithelium and experimentally 

 introduced foreign bodies, similarly receive a mucin coat as they 

 pass along the tube. Deposition is evidently continuous, so that on 

 entry into the uterus eggs often carry a mucin layer the thickness 

 of which is equal to or greater than the original diameter of the 

 egg, including the zona pellucida; in other words, the overall 

 diameter of the egg undergoes at least a threefold increase (Fig. 10). 

 The thickness of the mucin coat was reported to be diminished by 

 the injection of oestradiol into the rabbit (Green wald, 1957) and 

 increased by the injection of progesterone (Greenwald, 1958); 

 Noyes, Adams and Walton (1959), on the other hand, found that 

 mucin deposition was not prevented by ovariectomy and might, in 

 fact, be increased by the administration of small doses of oestrogen 

 to ovariectomized rabbits. The last-named authors consider that the 

 thickness of the mucin coat depends more upon the time spent by 

 the egg in the mucin-depositing regions of the tube than upon 

 variations in the secretory activity of the tubal epithelium. 



The material constituting the mucin layer has been characterized 

 as a strongly acidic mucoprotein (Braden, 1952; Bacsich and 

 Hamilton, 1954). It was found to be digestible by trypsin, chymo- 

 trypsin and pepsin, but not by mould protease; it was insoluble 

 through the pH range of 2-0 to 9-0 and soluble in more alkaline 

 media than this; it was dissolved by hydrogen peroxide, with or 

 without ascorbic acid, but not by urea solutions or a variety of 

 oxidizing and reducing agents (Braden, 1952). Permeability studies 

 have shown that the mucin coat, like the zona pellucida, permits 

 the passage of dissolved substances of m.w. 1,200 or less (Austin and 

 Lovelock, 1958). 



The mucin coat is impenetrable to spermatozoa and its deposition 

 has therefore been said to limit the fertilizable life of the rabbit egg 

 (Pincus, 1930; Hammond, 1934). The time that deposition begins 

 has been variously put at 5 hr after ovulation (Pincus, 1930), 6 hr 

 (Hammond, 1934), not more than 8 hr (Braden, 1952) and 10 to 

 14 hr (Chang, 195 id, 1955c). The range in estimates may be owing 

 to the fact that they are based on observations on mated animals, 

 in which cumulus dispersal would have been expedited to varying 

 degrees by the hyaluronidase released from spermatozoa at the site 

 of fertilization. In unmated animals, cumulus dispersal is much 



