122 FERTILIZATION 



interesting review by Kushner (1954). Kushner reports that if 

 hens are inseminated on consecutive days by different and un- 

 related cocks, their offspring exhibit a higher growth rate, a higher 

 live weight, a higher blood haemoglobin content, and more vigour 

 than control chicks obtained by 'normal' insemination. This is 

 interpreted as being caused by 'heterologous' polyspermy (though 

 only one male pronucleus fuses with the female pronucleus) and 

 not, as might be thought, by heterosis. The supernumerary 

 spermatozoa which enter the egg are said to influence its meta- 

 bolism,* though it is also stated that the offspring resulting from 

 mixed inseminations on occasions exhibit characteristics of both 

 fathers. Similar experiments on pigs, sheep, cattle, trout, silk- 

 worms and silver foxes are reported, though in these cases, the 

 accent is more on increased fertility and better post-natal perform- 

 ance after insemination with semen from different sires than on 

 the appearance in the offspring of characteristics derived from 

 both male parents. But it is almost as difficult to understand why, 

 if one inseminates an animal with a mixture of semen from two 

 sources, the offspring should be more vigorous than those obtained 

 by insemination with either sort of semen singly, as it is to under- 

 stand the phenomenon of tri- or poly-parental inheritance. Double 

 inseminations would, of course, increase the number of sperm- 

 atozoa in the female reproductive tract and this might increase the 

 probability of fertilization. But this has nothing to do with the 

 subsequent performance of the offspring unless polyspermy is a 

 normal and, from the point of view of the future offspring, a 

 valuable event. Similarly, it is possible, as Kushner says, that 

 double matings increase the number of eggs ovulated, which might 

 cause increased litter sizes in pigs. But again, in the absence of 

 polyspermy, this has nothing to do with the later performance of 

 the offspring. 



Kushner also reports that the adverse effects of excessive in- 

 breeding in rabbits can be counteracted by adding bull or ram 

 semen to rabbit semen before its introduction into a doe. 



It is hardly necessary to mention that these experiments, if con- 

 firmed, must have a profound effect on the whole subject of fer- 

 tilization, quite apart from their impact on agriculture. The im- 

 portant thing is to repeat the experiments, which, in themselves, are 



* Warburg (iQii) and Brachet (iQ34/») have reported that the O2 uptake of 

 polyspennic eggs is slightly higher than that of monospermic eggs. 



