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This essay advances the theory that an unsuspectedly large part of embryonic death is attributable to genetic causes. The genetic factors involved are not necessarily inherited by the parents, indeed the majority probably arise de novo in each parent generation and some are likely to arise in the definitive gametes. The theory seeks to account for the nature of unexplained fertility differences between males, and of idiogenic infertility, and of some of the decline in fertility that is associated with age. It suggests that a considerable part of embryonic death is unavoidable and should be regarded as a normal way of eliminating unfit genotypes in each generation.

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1. Certain characteristics of bull semen have been examined at four artificial insemination centres in England. One hundred and twenty-one samples of semen, comprising 168 ejaculates from seventysix bulls of seven breeds, were examined. The semen from 119 tested samples was subsequently used to inseminate 4604 cows.The following semen characteristics were studied: volume of ejaculate; concentration of spermatozoa; incidence of dead spermatozoa; incidence of morphologically abnormal spermatozoa; initial fructose concentration; resistance of spermatozoa to temporature shock; methylene-blue reduction; O2 uptake; fructose utilization; visual evaluation of motility; impedance change frequency; and fertilizing capacity (conception rate).A clinical examination was made of the reproductive organs of each bull.2. Variations in the concentration of living spermatozoa and in the incidence of dead spermatozoa were found to account for most of the variations in the metabolic activity of semon: they also largely explained differences in physical activity as assessed by visual estimation of motility or by impedance change frequency.3. Although the rates of methylene-blue reduction, O2 uptake and fructolysis showed a close correlation with the numbers of living spermatozoa present, O2 uptake and fructolysis per living cell decreased with increasing cell concentration.4. Variations in metabolic activity showed no correlation with variations in the morphological characteristics of the spermatozoa, except in the case of spermatozoa with bent tails. These forms occurred in some samples as a result of staining treatment, but their incidence was positively correlated with the O2 uptake and fructolysis per living cell.5. There was evidence that highly active spermatozoa are more resistant to temperature shock than less active spermatozoa.