FERTILITY 657 



domestic animals is limited by the number of discharged and 

 fertilised ova which develop rather than by the number of ova 

 which are shed. With regard to the cause of the foetal atrophy 

 Hammond has shown that it is not due to bacterial infection 

 since the uteri are aseptic. He suggests that it may be brought 

 about by adiposity, by in-breeding, or by a genetic " lethal factor " 

 such as Kirkham ^ and others have shown probably to be the case 

 with the homozygous yellow mice which are never brought forth 

 alive but die irt. utero during implantation. The foetal atrophy is not 

 due to lack of room in the uterus, for this organ is capable of great 

 expansion, and the distribution of the degenerate foeti^ses is irregular 

 and does not suggest that death was due to overcrowding. It is 

 true, however, as Hammond has shown in pigs, that the size of the 

 foetus is roughly proportional to the extent of its membranes, and 

 that the festal membranes of single lambs, which are as a rule larger 

 than twins, extend to both horns of the uterus. It is also generally 

 true that the average weight of the embryo increases as the size of 

 the litter decreases, and that the larger and better nourished the 

 mother the larger are the young. Nutrition is probably an important 

 factor both in the size of the litter and in the sizes ef the individuals, 

 as experiments on various animals have shown, but there must be 

 variation in the degrees of vitality inherent in the different foetuses. 

 Some perish more easily than others as is well shown in pigs, but 

 even those which survive long enough to be born are often ill- 

 nourished and under-sized £fnd in striking contrast to other piglings 

 of the litter. Hammond states that atrophy of the foetuses begins 

 in the blood-vessels, which first become congested and then break 

 down. The foetal membranes may remain alive for some days after 

 the embryos have perished. 



The Increase of Tektility, a Pkoblem of Pkactical Breeding 



Heape ^ has shown from statistical evidence that the amount of 

 money invested in live stock in this country cannot be computed 

 at very much less than £450,000,000, and this sum does not include 

 the enormous capital spent on buildings, land, vehicles, and various 

 accessories. The annual export of live stock from Great Britain in 

 recent years has been tending steadily to increase, until it has 

 reached a total value of £1,750,000. It is clear, therefore, that 

 in this country the breeding industry occupies a position of no 

 inconsiderable importance, and that the scientific study of the 

 problems of breeding possesses a great national interest. Poremost 



' Kirkham, "The Fate of Homozygous Yellow Mice," Jour. Exp. Zool. 

 vol. xxviii., 1919. Lethal factors have been described for various other organisms 

 (^Drosophila, maize, etc.). See Babcock and Clausen, loc. cit. 



'^ Heape, The Breeding Indiistry, Cambridge, 1906. 



