132 DISCUSSION 



to remember that we injected hybrid material into maternal-strain 

 recipients. 



G. Klein: If it is really true that the trophoblast does not contain 

 antigens of paternal origin, even if the nuclei are haploid the assump- 

 tion that they are of zygote origin is quite untenable, because there is 

 just no cytogenetical mechanism by which the maternal genes would 

 segregate out from the zygote in their entirety at the time the tropho- 

 blast is formed. But I am wondering whether the negative result of 

 the immunization experiment really does exclude the presence of 

 paternal antigens, and whether you have made any attempt to show by 

 absorption tests or haemagglutination inhibition that the paternal 

 antigens are really absent. 



Hasek: We did not try other techniques to confirm the absence of 

 paternal-strain antigens in the hybrid placentas. And I would agree 

 that a cytogenetical mechanism for the recruitment of haploid cells of 

 foetal placenta containing only the maternal set of antigens, is rather 

 obscure. 



Brent: I am still rather worried about this question of contamination 

 of the placentas with maternal blood cells. The experience I have had 

 with the kind of test sytem Dr. Hasek has used, in which one examines 

 6- or 7-day-old grafts for accelerated destruction, leads me to beHeve 

 that it is rather difficult to make it as precise a quantitative measure as 

 one would like, and that the difference in degree of sensitivity in a 

 group of mice showing, say, 25 per cent survival of the graft epithehum, 

 and that of another group showing 50 per cent survival, is very small. 

 Couldn't such a relatively small difference be accounted for by the 

 presence of maternal leucocytes in Dr. Hasek's placentas? It is, I 

 imagine, fairly easy to wash off red cells, but not leucocytes, which 

 would stick to the capillaries and to the cells on the trophoblast. 



Woodruff: It strikes me that there are important resemblances be- 

 tween the barrier Dr. Billingham discussed in his paper, the barrier 

 Dr. Hasek has just talked about, and the artificial barriers used in 

 diffusion chamber experiments which are permeable to large molecules 

 but not to cells. 



Indeed, my interest in the immunological problem of pregnancy 

 arose out of diffusion chamber experiments which suggested that grafts 

 isolated from contact with host cells failed to immunize. Our Chair- 



