DISCUSSION 135 



some of the ectoplacental cone grafts, possibly because of contamination 

 with embryonic fragments during the surgical division of the concep- 

 tus, it was felt that further efforts to obtain a pure preparation of 

 trophoblast for grafting were warranted. 



This was achieved by harvesting fertilized eggs (again (A x CB A) Fj) 

 from the oviducts two days after the appearance of the copulation plug 

 and again implanting them beneath the kidney capsule of the recipient. 

 In many instances these tiny structures, composed of from 2 to 8 cells, 

 promptly developed into small tumours which were made up altogether 

 of trophoblastic giant cells. These grafts survived in excellent condition 

 for at least 12 days while provoking no cellular reaction. The tropho- 

 blastic cells remained in intimate contact with the homologous re- 

 cipient and often infiltrated renal tubular cells. No specific sensitivity 

 of these recipients was revealed by later skin grafting with skin of 

 donor origin. Most striking was the fact that exactly the same vigorous 

 development and survival of trophoblast could be readily procured on 

 grafting fertilized eggs to animals which had recently rejected donor 

 type skin grafts, in fact even after second-set skin grafts (Fig. 4). 



The trophoblast occupies an intermediary position between the 

 maternal and foetal circulations as a zone to which never less than two 

 or three cells contribute at any single point (see the electron micro- 

 graphic studies in the rat of G. B. Wislocki and E. W. Dempsey [1955. 

 Anat. Rec, 123, 33]). We have interpreted our experiments (to be 

 reported at length elsewhere) as giving support to the idea that the 

 trophoblast may act as an immunological buffer zone in the mouse. It 

 is not only properly situated anatomically, but it also appears to fulfil 

 the requirement of being less able to provoke a reaction on the part 

 of the host than other portions of the conceptus. 



Silvers: I would like to return to the question of parity and tolerance, 

 and present some evidence which suggests that this tolerance is dis- 

 tinguishable from the tolerance which results from a neonatal inocu- 

 lation of cells. 



C57BL females rendered tolerant of male isografts following their 

 inoculation at birth with isologous male cells are, indeed, cellular 

 chimeras. Thus, as Dr. D. Wilson in our department has shown, 

 normal females following parabiosis with such tolerant females them- 

 selves become incapable of rejecting male isografts. Furthermore, the 



