146 INTRODUCTION TO SEXUAL PHYSIOLOGY 



The occurrence of foetal atrophy and its possible causes have 

 abeady been discussed. 



Eate of Propagation 



Among amphibians, fishes, and a very large number of in- 

 vertebrates, probably the majority of the ovarian eggs are 

 actually spawned, and here the rate of propagation is regulated 

 by the number of eggs which become fertilised and, escaping 

 the many dangers to which they are exposed, are able to com- 

 plete the process of development. Thus it is stated that the 

 cod spawns six million eggs, less than a third of which become 

 fertilised. In the higher animals, however, as well as many 

 of the lower, only a certain proportion of the potential eggs 

 formed in the ovary ever reach maturity at all, and a still smaller 

 percentage are released from the organ so as to obtain a chance 

 of becoming fertilised. Balfour was the first to show that in 

 the rabbit one embryonic egg may develop at the expense of 

 others, and that the eggs which disappear may serve as food 

 material for the one ovum which, owing to a superior vigour or 

 to some chance circumstance relating to its position in the ovary, 

 was able to survive. Thus, there is a veritable struggle for 

 existence amongst the gametes during the process of develop- 

 ment within the gonads, and those gametes which survive may do 

 so by taking advantage of the death of others at a very early 

 stage of existence. Aral has estimated that the ovary of the 

 rat at birth contains 35,100 ova, but that these are reduced by 

 degeneration to 11,000 after twenty-three days, and to 6,000 by 

 the sixty-third day. We have seen already that nutrition plays 

 an important part in regulating the proportion of the eggs which 

 survive, so as eventually to become mature. 



The Birth-Rate in Man 



Statistics show that the birth-rate (that is, the proportion 

 of children born annually to the total number of the population) 

 in civilised countries has for some years past been declining. This 

 is true not only of the countries of Europe but in most parts of 

 the Western hemisphere where European races have settled, 

 as well as in Australia and New Zealand. In France the actual 

 population has been approximately the same for the last three 



