EVIDENCE FOR THE DEATH IN UTERO OF THE 

 HOMOZYaOUS YELLOW MOUSE^ 



HEMAN L. IBSEN AND EMIL STEIGLEDER 



CuENOT (1905) and Castle and Little (1910) have pre- 

 sented conclusive evidence that yellow mice are always 

 heterozygous and hence cannot be made to breed true. 

 Their combined results show that when yellows are 

 mated together the proportion of yellows to non-yellows 

 in the offspring is almost exactly 2 : 1 instead of the usual 

 3:1 ratio resulting from the mating of heterozygotes. 

 Castle and Little seem justified on this account in assum- 

 ing that the homozygous yellows are not viable, especially 

 since the size of litter from the yellow X yellow mating 

 is markedly smaller than that obtained from yellow X 

 non-yellow or non-yellow X non-yellow matings. 



Until quite recently no attempt had been made to de- 

 termine embryologically the actual fate of the homozy- 

 gous yellows. Since the present investigation was begun, 

 however, Kirkham (1917) has published a preliminary 

 statement of the results of such a study. His results, 

 presented only in abstract, show that of 69 embryOs from 

 yellow X yellow parents, 26 or 37.8 per cent, were de- 

 generating. For ''non-yellows" he used albinos.^ Of 



1 Papers from the Department of Experimental Breeding, Wisconsin Agri- 

 cultural Experiment Station, No. 11. Published with the approval of the 

 Director of the Station. 



[The problem of the fate of the homozygous yellow mouse was under- 

 taken at my suggestion during the summer of 1916 by Mr. Steigleder and 

 the experimental work on which the present paper is based was done en- 

 tirely by him. As he was, however, unable to complete the problem the 



checked all records with the preserved embryos and is alone responsible for 

 the tabulation, interpretation and presentation of the results.— L. J. Cole.] 



have carried the factor for yellow. From the fact that the proportion of 

 dead embryos was markedly different we may accept Kirkham 's assump- 



