252 PHYSIOLOGIC GENETICS 



the F 9 generation to 38 per cent in generations Fj to F 5 . 565 - 560 However, the tumors 

 appeared at a much greater age in the C3Hf than in the C3H. This was an interesting 

 result in that it demonstrated that although the virus could be a very important factor 

 in the etiology of these tumors, it was not essential when there was a strong genetic 

 susceptibility and the females were bred to add hormonal stimulation. 



Foster nursing alone, however, does not guarantee freedom from the virus because 

 of the possibility of transmission through the fertilized egg or the placenta, so subsequent 

 testing is essential. Filtrates of the C3Hf tumors were prepared and tested by injection 

 into agent-free test females and the results were negative, but these tests cannot be 

 considered absolute proof of the absence of the agent because in the preparation of the 

 filtrates viruses can be lost. Probably the best test for the presence or absence of the 

 agent is in the occurrence of tumors in females of subsequent generations. Throughout 

 the 45 generations of this subline, the tumors have continued to appear with a relatively 

 low incidence and at an advanced tumor age in females scattered throughout the 

 pedigree chart rather than being segregated in tumor families. We consider this to be 

 proof that the mammary-tumor agent is not present in this line. When the agent is 

 introduced into females of the line in even small amounts by injection or transmission 

 by the male or by any other way, it is immediately built up so that the females of 

 subsequent generations appear as a high tumor line comparable with the original C3H. 



Sometimes it is desired to have a large number of foster-nursed animals rather 

 than to establish a fostered line. To compare a mutated strain of the mammary- 

 tumor agent with the original agent, Blair 109 found it necessary to foster nurse a large 

 number of offspring that had not been permitted to nurse upon their mothers. To do 

 this she devised a screen on which the young were born and immediately dropped 

 through to the foster mother below. 



Sublines can readily be developed through transfer of fertilized ova, which 

 eliminates not only possible agents transferred through the milk but also those that 

 might be transferred through the placenta. This is a delicate procedure, and 

 Dr. Deringer gives a full description of it in the appendix of this volume. Dr. Deringer's 

 subline C3He produced by transfer of fertilized ova from C3H to C57BL has proved very 

 interesting in that, like the C3Hf, it continues to produce mammary tumors although 

 at a lower incidence and greater age than the C3H with the agent. 257 



A third type of subline can be developed from transplanted ovaries. Ordinarily, 

 however, transplantation is possible only between histocompatible animals such as 

 those of two isogenic strains, if the gene by which they differ is not an histocompatibility 

 gene, or from animals of a parental inbred strain to its F x hybrid. Recently we have 

 made reciprocal transplants of ovaries between the yellow and the black agouti (C3H x 

 YBR) F x hybrid females. These F x hybrids are alike except for the difference at the 

 agouti locus and possibly closely linked loci. The operation was simple with dorsal 

 incisions and immediate transfer of the ovaries to the capsules of the reciprocal female. 

 Agouti females bearing yellow ovaries when mated to agouti males gave birth to 

 successive litters containing yellow offspring, proving that the transplanted ovaries 



