FERTILITY 139 



in xdero than by the production of ova. In such animals the 

 number of eggs matured is frequently in excess of the nutrition 

 available for them, and this leads to atrophy as newly fertilised 

 ova or partially developed embryos (Hammond). Foetal atrophy 

 has also been described in certain wild animals (rat, mouse, etc.), 

 but is uncommon. 



Hammond has shown that foetal atrophy cannot be due to 

 bacteria] infection since the uterus is aseptic. Neither is it 

 due to overcrowding in the uterus, since this organ is capable 

 of great expansion ; moreover, the distribution of the degenerate 

 foetuses is irregular and does not suggest that death is due to 

 overcrowding. Hammond supposes that the phenomenon may 

 be due to adiposity, or to inbreeding, or to a genetic " lethal 

 factor " (that is, something inherent in the embryo itself), in 

 the same kind of way as Kirkham found with the homozygous 

 yellow mice which are never brought forth alive, but die in 

 utero during implantation.^ 



VlTAMINES AND FERTILITY 



It is now^ known that sterility may be due to the absence 



of vitamines or accessory nutritive substances, the presence of 



which is essential for normal metabolism, undergrowth or. 



" deficiency diseases," such as scurvy or beri-beri, occurring when 



one or other of the vitamines is absent.' Evans and Bishop 



claim further there is a vitamine present in green food which has 



a specific connection with the generative processes, and that in 



the absence of this vitamine foetal absorption occurs. Further, 



Parkes and Drummond have shown with rats that in the absence 



of the "water-soluble vitamine B" (the deficiency of which is 



associated with "beri-beri"), neither sex is able to produce 



gametes, so that the animals are completely sterile. It is 



possible that some of the cases where captivity or a change of 



environment appear to cause sterility may be accounted for by 



the absence of the necessary accessory food substances from 



the diet. 



i Hybrid yellow mice of a certain type when interbred slioukl produce, 

 according to Mendelian expectation, one pure non-yellow {e.g. black), 

 two hybrid yellow, and one pure yellow. Actually only two yellows 

 are produced for one non-yellow, the pure or homozygous yellows dying 

 before birth. Lethal genes have also been found in the vinegar fly, 

 Drosophila, as well as in a breed of cattle. 



