130 DISCUSSION 



notion that the trophoblast derives from the granulosa cells surrounding 

 the ovum and doesn't come from the zygote. This has been suggested 

 and totally discarded, I think, on the grounds that there were no 

 granulosa cells to develop and that in organ culture work you only got 

 development of trophoblast if you had removed everything including 

 the zona pellucida. This type of difference that you have described 

 might be used to support the notion. There is also the suggestion that 

 the trophoblast cells may be haploid. I believe someone has estimated 

 the amount of DN A in trophoblast cells and found that they contain 

 half the expected amount; I don't know how much credence should be 

 put in that. And there may be a difference in the behaviour of haploid 

 cells as compared to the diploid ones. 



Barrett: I agree, in a modified form, as to the antigenicity of the 

 placenta itself. Our test was a little different from either Dr. Hasek's or 

 Prof. Krohn's. We used the hybrid combination B ALB/c x DBA, as in 

 our first report. We gave one hybrid placenta, washed free of blood, as 

 a subcutaneous dose to a maternal-strain recipient, and we gave the 

 foetus at 14 days and at 12 days as a single subcutaneous dose. This of 

 course was a haploid dose of the male antigen. Each tissue produced 

 transplantation immunity in the maternal strain as tested by the subse- 

 quent inoculation of the specific tumour. However, the embryo in- 

 duced a good strong immunity (allowing for this dosage variation) ; the 

 placenta induced a significant and perfectly adequate immunity, but it 

 was not as strong as that induced by the foetus. So here again is the 

 slight strain variation that you mentioned, here again is the slightly 

 different dosage — I don't know what a placenta would weigh, but I 

 would think it would weigh 300-400 mg. 



Hasek: There are 3 placentas for a dose of 250 mg. 



Barrett: Then our dosage was smaller. As to the effect of difference in 

 strain Prof Medawar mentioned, we have recently published {ig6i.J. 

 nat. Cancer Inst., 27, 409) results that bear on this. In our original report 

 the females were all B ALB/c, and they were mated to B ALB/c, to 

 DBA, and to C3H. When we tested with the DBA tumour, specific 

 tolerance was found in only those mated to DBA. Now we have 

 found that if we introduce the C3H tumour then, as expected, it is the 

 animal mated to the C3H male which shows tolerance. And we are 

 getting outside the H-2 barrier. 



