216 THE PHYSIOLOGY OF REPRODUCTION 



results of Castle and Morgan have been repeated by Fuchs, 1 who 

 finds that, in Ciona, the self-fertilisation rate can be greatly increased 

 if concentrated sperm suspensions are employed. This rate can also 

 be much increased if a little egg extract is added to the sea-water 

 in which the fertilisation is carried out. The presence of the egg 

 extract has an immediate effect in raising the fertilisation percentage. 

 Extract of ovary or blood acts in the same way, the movements of 

 the sperm being greatly stimulated. The sperm of the sea-urchin 

 Strongylocentrotus is stimulated in the same manner by egg extract of 

 its own eggs or those of Sphcereckinus, Echinus, or Ciona eggs, but 

 Asterias egg extract completely inhibits the fertilising power of a 

 Strongylocentrotus sperm suspension. Fuchs has shown that a small 

 rise in the H-ion concentration of the sea- water brings about much 

 the same effect as the addition of egg extract, and, as a matter of 

 fact, the addition of egg extract to normal sea-water caused a slight 

 rise in its H-ion concentration. 



Cohen 2 lias shown that in sea- water in which the H-ion 

 concentration has been slightly raised above the normal, the life of 

 the sperm is greatly prolonged, and they are able to fertilise a 

 greater number of eggs. 



It is well known that the fertility of animals which are much 

 in-bred is often reduced, but this is by no means invariably the case. 3 

 Thoroughbred horses are notoriously in-bred, and it is interesting 

 to note that one of the earlier Reports of the Royal Commission on 

 Horse-breeding states that no less than forty per cent, of the 

 thoroughbred mares in this country fail to have foals each year. 

 This relatively large amount of sterility is probably due to a variety 

 of causes, and not entirely to the result of in-breeding. 4 



Low 5 has recorded an experiment on the effect of in-breeding in 

 fox-hounds. The particular strain is described as having perished 

 completely. Low states also that similar experiments have been 

 performed upon pigs, and, as a consequence, the litters became 

 diminished in size and frequency, while difficulty was often 

 experienced in rearing those which were produced. 



1 Fuchs (H. M.), " Studies in the Physiology of Fertilisation," Jour, (lenetics, 

 vol. iv., 1915. 



2 Cohen (E. J.), "Studies in the Physiology of Spermatozoa," Biol. Bull., 

 vol. xxxiv., 1918. 



3 The results of in-breeding are discussed at some length by Darwin, 

 Variation of Animals and Plants, vol. ii., Popular Edition, London, 1905. For 



reviews of the subject see Morgan, Experimental Zoology, New York, 1907 ; 

 and East and Jones, In-breeding and Out-breeding, Philadelphia and London. 

 Pearl has worked out a mathematical formula for estimating the degree of 

 in-breeding, Amer. Nat., vol. xlvii., 1913. 



4 Returns obtained by the Ministry of Agriculture show about the same 

 degree of fertility for cross-bred mares served by Shire stallions. 



5 Low The Domesticated Animals of Great Britain, London, 1845. 



