NUTRITION IN RELATION TO REPRODUCTION             205

breeding efficiency. In a series of experiments on rats, Slonaker and Card (62,
63, 64) showed that the age at puberty, the length of the reproductive span and
the size of litters were all affected by the quality of the protein fed. Rats
receiving animal protein matured earlier than those receiving only vegetable
protein; the intervals between litters were about two months in the former
and varied up to four months in the latter. The period of reproductive
activity on animal protein was seventeen months ; on vegetable protein only two
to seven months. Sterility was high on vegetarian diet; the number of young
in litters was greatly reduced and the effect was cumulative, the vegetarian rats
being completely sterile in the third generation. Simmonds (44) showed, that a
diet in which only vegetable protein was supplied greatly reduced fertility, while a
high proportion of animal protein increased it. Wan and Wu (65) also found a
significant difference in litter size in favour of omnivorous, as compared with
vegetarian, rats. Later, Slonaker (66) showed that there is an optimum protein
level for reproduction at 1.4 per cent. of the ration (animal protein), higher or
lower levels giving diminished litter size and increased sterility. This level was also
best for length of reproductive span. A large excess of animal protein over the
optimum level causes pathological changes in the ovary and promotes sterility (67).

The difference in value between animal and vegetable protein for reproduction
may possibly be due to deficiency of essential amino-acids in the latter. Courrier
and Raynaud (68) have shown that rats fed on a diet deficient in lysine showed no
sexual cycle. Males on the same diet had relatively poorly developed testicles.

In poultry feeding it is well recognized that sufficiency of calcium and phospho-
rus and an adequate supply of good quality protein are essential for egg production.
Optimum quantities and ratios are being determined by experiments at all the
main centres of research (69, 70, 71). According to the most recent results,
plant protein, adequately supplemented with calcium and sodium chloride, may give
as good results as animal protein. There are, however, no data to show whether
this is true for mammals.

Vitamin deficiency.—The vitamin most clearly connected with reproduction is
vitamin E (72). This vitamin is fat-soluble, and occurs in nature, chiefly in green
vegetables, notably lettuce, and in the embryos of certain cereals, especially wheat.
It is found in milk and egg-yolk, and is present in muscle, fat, pituitary, placenta,
and to a less extent in intestine and liver. It does not occur in plant oils, and is
apparently absent from cod-liver oil, kidney, brain and testis. A diet free from
vitamin E does not prevent maturation or shedding of ova in rats, and implanta-
tion is normal. A first litter may even be born, but as depletion of reserves pro-
ceeds, a characteristic condition with death and absorption of the foetus results.