THE EARLY EMBRYOLOGY OF THE MOUSE 39 



are larger than elsewhere in the decidua, and this together with the grouping 

 of the nuclei gives the zone a ver>^ characteristic appearance. It will be 

 noted that it lies between the embryo and the new uterine lumen. With the 

 growth of the embryo it becomes stretched until, in the later part of the 

 gestation period, it is hardly more than a thin membrane separating embryo 

 and lumen. Dorsally there is a mesometrial zone, or decidua basalis. whose 

 cells at 8 days still closely resemble those of the unaltered mucosa. It later 

 contributes to the formation of the placenta. Between the antimesometrial 

 and mesometrial zones is an intermediate or vascular zone characterized 

 by the presence of numerous irregular endothelial-lined blood spaces or 

 sinusoids. Its cells tend to be multi-nucleate like those of the decidua 

 capsularis. 



The nourishment of the embryo. — The source from which the embryo 

 derives its nourishment during its earliest growth period is somewhat uncer- 

 tain, but it is not unlikely that the degenerating cells of the uterine epithe- 

 lium that originally lined the implantation chamber serve as a source of food. 

 The epithelium is sloughed ofif and begins to undergo degenerative changes at 

 just about the same time that the first real increase in size of the embryo is to 

 be noted. At the mesometrial pole of the embryo at 4^2 days may be seen 

 droplets of secretion that contain perhaps an enzyme concerned with the 

 digestion of the epithelial cells (Fig. 6). This stage in the nourishment 

 of the embryo is brief; by 5I2 days only a remnant of the epithelial cells 

 remains (Fig. 8). 



At the same time a new source of nourishment makes its appearance. It 

 has already been stated that the intermediate zone of the decidua contains 

 numerous blood-filled sinusoids. At 5^2 days these begin to rupture, pour- 

 ing their contents into the lumen surrounding the embryo. In a very short 

 time the embryo is completely bathed in maternal blood. It has recently 

 been shown that this blood is not stagnant as was once supposed, but that it 

 remains a part of the maternal circulation. In certain experiments with the 

 rat it was found that there is a complete replacement every twenty minutes 



(13). 



The maternal blood is separated from the embryo proper by Reichert's 

 membrane, the yolk cavity and, in later stages, by the yolk-sac. Reichert's 

 membrane probably plays an entirely passive role in the transportation of 

 nutrient substances from the maternal blood to the embryo, acting simply as 

 a semi-permeable membrane. The yolk-sac, on the other hand, probably 

 actively absorbs the food material. This is particularly true after the blood 

 islands which girdle the yolk-sac have developed into a capillary network and 



