THE EARLY EMBRYOLOGY OF THE MOUSE 37 



At 73<4 days the extra-embryonic splanchnopleure or yolk-sac is a struc- 

 ture of limited area forming the central or ectoderm free portion of the egg 

 cylinder wall (Fig. 19A). While small at first, it is an area of rapid growth 

 and by 8 or 8 V2 days forms an extensive membrane enveloping the amnion 

 and a greatly enlarged exocoelomic cavity (Figs. 19B and C). The whole 

 embryo changes its shape in the process, the egg cylinder becoming an ovoid 

 and the ovoid a sphere. At 8 days the yolk-sac is still attached to the 

 embryo along a band that runs anterior to the opening of the fore-gut and 

 posterior to the opening of the hind-gut, so that most of the ventral surface 

 of the embryo is outside it (Fig. 22). After the mid-gut has formed, how- 

 ever, this portion of the embryo, too, is enveloped by the yolk-sac (Fig. 20). 

 The details of this process will be described later. 



The blood islands. — Associated with the yolk-sac splanchnopleure in all 

 species in which it occurs are structures known as the blood islands. These 

 appear in the mouse at 7)^ days as thickenings in the inner or mesodermal 

 layer of the yolk-sac about which they form an irregular girdle (Fig. 16). 

 As the name implies, the blood islands give rise to part of the circulatory 

 system. The peripheral cells differentiate to form the endothelium of a 

 system of blood vessels encircling the yolk-sac while the inner cells become 

 primitive blood corpuscles that circulate in the embryonic blood stream. 



Changes in the uterus. — Implantation is accompanied by a rapid growth 

 of the uterine mucosa adjacent to the implantation site to produce a definite 

 swelling, the decidual swelling. For a while the uterine crypt containing 

 the embryo maintains its connection with the uterine cavity, but by about 

 7}^ days the growth of the decidua has blocked this off so that the cavity 

 containing the embryo is separated from the main lumen (Fig. 4B). The 

 bridge of tissue thus formed dorsal to the ectoplacental cone will later become 

 part of the placenta. Further growth of the decidua constricts and finally, 

 by about 8 days, completely closes the uterine lumen dorsal to the embryo 

 except for one or more small isolated chambers (Fig. 21). On each side of 

 the decidual swelling the uterine lumen remains open, but at this period in 

 development there is no continuous passage throughout the length of the 

 uterus. A little later a continuous lumen is reestablished, but the new 

 kimen is on the opposite side of the decidual swelling from the old, passing 

 ventral instead of dorsal to the embryo. An early stage in this reestablish- 

 ment of the lumen, may be seen at about 8 days (Fig. 21). The epithelium 

 lining the lumen on each side of the decidual swelling has grown in between 

 the muscle layers and the decidua ventral to the embryo. The extreme 

 limits of this growth consist of a double but unsplit layer of epithelium. In 



