FERTILIZATION AND CLEAVAGE 87 



While Hammond's deductions are entirely reasonable, it 

 is possible that the 6 hours of fertilizable life allotted to 

 rabbit ova is possibly too short since in normal matings 

 13^ to 3 hours are required by the sperm to reach the ova. 

 This would make the critical period some 73^ to 9 hours 

 long. Furthermore it is not the arrival of the first sperm 

 that is effective, since as we have previously seen (pages 77 

 to 78) a definite minimal sperm concentration is necessary 

 for both folUcle cell dispersion and fertilization. If the 

 critical period were thereby further lengthened by 1 to 

 2 hours it would coincide almost exactly with the time 

 when the ova separating out of the tubal plug begin to ac- 

 quire a coating of albumen. This coating is impervious to 

 sperm (Pincus, 1930). 



Similar experiments of Hammond and Walton (1934) with 

 the ferret show that fertile matings made as late as 30 hours 

 after ovulation result in the production of young. The rea- 

 sons for the maintenance of the fertilizing capacity of ferret 

 ova for as long as 30 hours are not deducible in detail since 

 the exact tubal history of ferret ova is not known. Hammond 

 and Walton attribute the greater length of fertilizable life 

 in this case to the longer time it takes for the ova to trav- 

 erse the oviduct, e.g., 5 to 6 days in. the ferret compared 

 with 3H days in the rabbit and the presumably correlated 

 slower dissolution of the plug of massed ova. 



In the spontaneously o\ailating mammals the fertilizable 

 life of the ova is also of short duration, but exact data are 

 not available since it is ordinarily difficult to ascertain 

 the specific time of ovulation. Hartman (1924) has shown 

 that opossum ova traverse the tubal portion of the oviduct 

 in 24 hours and that upon entry into the uterus unfertilized 

 ova are definitely degenerated. Charlton (1917) found clear 

 signs of degeneration in unfertilized tubal mouse ova by 

 two days after parturition. Since post-partum ovulation 

 occurs in the mouse at about 14 hours after parturition (Long 

 and Mark, 1911) mouse ova may be said to retain cyto- 

 logical normality for about 35 hours. In the rat ova present 



