FERTILISATION 209 



a general want of vigour. It seems probable, however, that it 

 also results from failure on the part of the gametes to conju- 

 gate, since the productiveness of in-bred animals can often be 

 increased by cross-breeding with other varieties. (See p. 601.) 



Heape l states that Dorset Horn sheep, when served by 

 rams of their own breed, show a greater tendency towards 

 barrenness than when served by Hampshire Down rams. It 

 is possible that what in this case appears to be barrenness is 

 in reality very early abortion, the in-bred embryos tending 

 to die at an early stage and to be absorbed in utero, thus 

 escaping observation. It seems not unlikely, however, that, in 

 the absence of cross-breeding, there is sometimes an insufficiency 

 of vitality at the very outset, the elective affinity of the gametes 

 being too feeble to induce conjugation. 



Some years ago the writer carried out an experiment upon - 

 a bitch belonging to the Dandie Dinmont variety, which is 

 known to be very in-bred. Seminal fluid was obtained from a 

 pure-bred Dandie Dinmont dog, and also from an obviously 

 mongrel terrier of unknown ancestry. The semen from the two 

 dogs was examined microscopically, and in each case was found 

 to be rich in sperms, which so far as seen were all moving and 

 in a vigorous condition. Approximately equal quantities of 

 each sample of semen were then mixed together in a glass tube. 

 After a further examination of the mixture, when it was 

 observed that all the sperms were still active, the fluid was 

 injected into a pure-bred Dandie Dinmont bitch, which was 

 distantly related to the Dandie Dinmont dog. Previously to 

 the experiment the bitch had been kept apart from other dogs, 

 and this restriction was continued so long as she showed signs 

 of cestrus. Fifty-nine days after the injection the bitch littered 

 four pups, which closely resembled one another. Of these one 

 died early, but the other three grew into mongrels which some- 

 what resembled the terrier sire, so that there can be little doubt 

 that all four pups were mongrels. No stress should be laid 

 upon the result of a single experiment ; but the evidence, such 

 as it was, was indicative of a selective tendency, consequent 

 upon a reduced vitality, on the part of the ova of the in-bred 



1 Heape, "Abortion, Barrenness, and Fertility in Sheep," Jour. Royal 

 Agric. Soc., vol. x., 1899. 



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