134 DISCUSSION 



Grafts of placenta, taken io|^ days after mating, and placed either 

 topically, on bare areas prepared on the thoracic wall, or intramus- 

 cularly, are promptly rejected in the presence of a dense mononuclear 

 cell infiltrate. Later A-strain skin grafts to these recipients are rejected 

 in accelerated fashion. The difficulty in interpreting experiments of 

 this kind in which placenta at this age is used, however, hes in the 

 important fact that at this stage the placenta is a composite structure 

 made up not only of trophoblast but also of vessels, with their con- 

 tained leucocytes, and a certain amount of supporting connective tissue, 

 all of embryonic origin. The frank sensitization which we demonstrated 

 could, then, be due to the embryonic components of the placenta at this 

 stage. 



We thus turned to earher embryonic stages. At yj days the mouse 

 conceptus is a roughly bullet-shaped structure measuring about o • 75 

 mm. in length. One end is made up entirely of trophoblast, the 

 " ectoplacental cone", since vascular ingrowth from the foetus has not 

 yet occurred. The other end is composed of developing embryo. It is 

 possible to separate these two portions, more or less cleanly, under the 

 dissecting microscope and to implant each one separately. This was 

 done, placing one portion under the renal capsule on one side and the 

 other under the renal capsule on the contralateral side of the same re- 

 cipient. In this experiment the recipients had recently been sensitized 

 to paternal antigens by A-strain skin grafts in order to accentuate any 

 reaction which might occur. Although the embryonic portion usually 

 grows and continues developing quite well after transfer to an isogenic 

 recipient (Fig. i), it is fully destroyed by seven days after transplan- 

 tation to previously sensitized CBA recipients. Nevertheless, one can 

 often distinguish small groups of viable trophoblastic giant cells 

 amongst the dense mononuclears of the host, where they are apparently 

 enjoying an anomalously prolonged survival (Fig. 2). In contrast to 

 this, ectoplacental cone grafts survive about as well whether transferred 

 to isologous or previously sensitized homologous hosts, with very 

 little cellular reaction on the part of the host (Fig. 3). Degeneration of 

 these grafts finally occurs after 12 to 13 days in both homologous and 

 isologous situations, however, as the cells appear to undergo a *' non- 

 specific" necrosis (Fawcett, D. W. [1950]. Anat. Rec, 108, 71). 

 Since shght degrees of cellular reaction had been observed against 



