Doe rabbits were inseminated with sperm suspensions containing a known number of morphologically normal sperms, together with a known number of tailless sperm heads, produced artifically by cold treatment of the scrotal testes. The fertilizing capacity of the mixture, as determined by the percentage of fertile inseminations and the average number of young born, remained normal so long as a minimal number of normal sperms (viz. 3 millions) was maintained, irrespective of whether a high percentage of sperm heads (viz. 17-95) was present or not. The isolated sperm heads had obviously no fertilizing capacity. Partial fertility occurred when the number of morphologically normal sperms was less than 3 millions and sterility when the number was less than 1 million.

When a similar of inseminations were she with spermatozoa from normal untreated males the minimal number of normal sperms required for full fertility was 0·33 million, or rather less. This indicates that apparently normal sperms which are present in ejaculates after low temperature treatment of the testis are only morphologically normal and not physiologically normal. It is concluded from these results that the diagnosis of fertility or sterility, based only upon sperm morphology, may be inaccurate and misleading. A seminal specimen containing a high proportion of abnormal forms may be quite fertile provided it contains a sufficient absolute number of viable sperms, and conversely, spermatozoa which may appear morphologically normal may not be physiologically capable of effecting fertility.

The sex ratio of the offspring was not significantly disturbed when does were inseminated with sperms produced after cold treatment of the testes. The efficiency of artifical insemination as a method of ensuring high fertility has been demonstrated.

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