40 BIOLOGY OF THE LABORATORY MOUSE 



after the embryonic circulation is established. By the time this occurs the 

 yolk-sac has become pressed against and partly fused with Reichert's mem- 

 brane, obHterating the yolk cavity (Fig. 20B). The embryonic yolk-sac 

 circulation is thus brought very close to the maternal circulation, and the 

 yolk-sac is established as "an organ of exchange whose importance is not 

 secondary to that of the allantoic placenta" (13). It is interesting to 

 observe that in the rodents the yolk-sac has thus recovered in full measure 

 the role as an organ of absorption which it possessed in the reptiles, with, 

 however, the important difference that the material absorbed comes from the 

 maternal blood instead of from yolk deposited within the egg. 



The sinusoids in the intermediate zone of the decidua extend from the 

 decidual cavity containing the embryo clear to the periphery of the decidual 

 swelling where this borders on the uterine lumen. Beginning at about 7 3^^ 

 or 8 days there is bleeding into the uterus from these peripheral sinusoids 

 (64, 66). At about 10 days some of this blood finds its way into the vagina, 

 persisting there for 3 or 4 days (57). It is a convenient early sign of 

 pregnancy. 



In the later stages of development the decidua basalis, the ectoplacental 

 cone, the chorion, and parts of the allantois fuse to give rise to a true 

 placenta which thereupon assumes a major role in transferring nutritive 

 material to the embryo. 



The giant cells. — A conspicuous feature in sections of mouse embryos of 

 6 to 14 days is the presence of certain remarkably large cells lying between 

 Reichert's membrane and the decidua. These are the so-called giant cells 

 (Fig. 21). Because of the early and close fusion of embryonic and maternal 

 tissues in the region which the giant cells later occupy, their origin is difficult 

 to determine and has been the subject of extended debate (3, 22, 48 and 

 others). Some authors believe that they are derived from the decidua, 

 others that they come from the trophectoderm. Their function likewise has 

 been the subject of much discussion. Our own observations, briefly pre- 

 sented below, seem to us to be fairly conclusive on a few points, but to leave 

 others still in doubt. 



It is convenient to distinguish three types of giant cells. The first large 

 and unmistakable giant cells to appear are at the ventral extremity of the 

 embryo (Figs. 6 and 8). They are quite evidently derived from the trophec- 

 toderm. Already quite large at 51^ days, they become, relatively speaking, 

 enormous by 7 days at which time they have penetrated for some distance 

 into the remains of the implantation cavity ventral to the embryo. These 

 are primary giant cells. The trophectoderm cells lateral to the egg cylinder 



