STERILITY, the inability to beget living offspring, may be the result of any of the following causes.
Absence of sexual intercourse due to the lack of appropriate psychological stimuli leading to coition. This is one factor in the mutual sterility of different species. (See HYBRIDISM.) Absence or incompleteness of coition due to gross abnormality of the external or internal organs of reproduction. Hermaphro ditism (see article, SEx) occlusion of the ducts which convey the gametes from the sex-gland to the site of fertilisation, either congenital or acquired through disease; imperforate hymen and other structural abnormalities fall into this group. The chemical condition of the female passages may be such as to kill the entering spermatozoa.
Infecundity.—The inability to produce functional gametes is found in many hybrids (see HYBRIDISM) and in certain intersexual forms. In such mammals in which the testes lie normally within a scrotum, the testis if retained within the abdominal cavity does not produce functional spermatozoa. Infecundity may follow upon exposure to X-rays, for these destroy that tissue of the sex-gland which is concerned with the elaboration of the gametes.
Mutual repugnance on the part of the gametes resulting in non-fertilisation. Though as yet nothing very definite is known of the physicochemical requirements of fertilisation there is a con siderable body of evidence to show that an attraction between the male and female gametes must exist if fertilisation is to occur.
Abnormalities in the fertilised egg which embarrass or prevent cleavage and development bringing these processes to a stop at some stage, death of the individual thereupon resulting. This is the fate of many hybrid embryos. (See HYBRIDISM.) Lethal
factors (see GENETICS and SEx) also operate in this fashion.
Abnormalities in the medium in which the fertilised eggs find themselves which prevent or embarrass development. The exact constitution of the sea-water in terms of alkalinity, for example, determines whether or not the fertilised ova of marine organisms shall begin and continue their development. Pathological con ditions within the uterus of the mammalian female can prevent proper implantation of the descending egg.
Disharmonies between the developing offspring and the mater nal environment can lead to difficult labour and death of either or of both mother and young. For example the male cattle X bison hybrid has a prominent crest and this cannot be accommodated by the maternal passages of the cow. Such male hybrids there fore usually are still-born. Contracted pelvis and similar abnor malities of the female birth-canal are common causes of death of both infant and mother. The general well-being of the mother is necessary for the normal development of the infant she bears.
In the case of the human subject there are certain conditions which are essential to fertility. Healthy spermatozoa must be received into a healthy vagina with a normal cervix, uterus, tubes and ovaries. An examination of any case of sterility must there fore begin with an examination of the male. It is estimated that in 25 per cent of childless marriages the man is at fault.
Experimentation with laboratory animals has shown that in fecundity may result from deficiency of the reproductive vitamin E (Mason, 1925).