THE PLACENTA. 



443 



Fig. 170. — A transverse section across the uterus and the contained embryo 

 of a Rabbit at the end of the nineteenth day. The embryo is cut trans- 

 versely, about the middle of the body, the section passing through the 

 yolk-stalk and allantoic stalk, x 3J. 



A, dorsal aorta of the embryo. AA, allantoic ai-tcry. AX, amnionic cavity, be- 

 tween the inner or true amnion and the embryo. CX, space between the inner 

 and outer layers of the amnion (c/. Fig. 148). G-U, uterine glands of obplacental 

 region. H, hypoblast of upper or vascular ^^■all of yolk-sac. MI, outer or longitudinal 

 muscles of the wall of the uterus. IVCX, inner or circular muscles of the wall of the 

 uterus. MM, mesometrium. liTS, spinal cord of the embryo. N'T, sympathetic nerve 

 cord. PH, lobule of placenta. PO, region along which the separation of the placenta 

 occurs at birth. PB^ interplacental groove. PAV, subplacental cavity. SI, sinus 

 terminalis. TA, cavity of the allantois (c/. Fig. 148). TS, stomach of embryo. TJC, 

 dilated uterine capillary, Avith thick perivascular wall. U P, uterin e or maternal sinuses 

 of placenta. U V, blood-vessels of uterus. VA, allantoic vein. "W, liver of embryo. 

 Y"K, yolk-stalk. YL, dotted line representing the lower or non-vascular wall of the yolk- 

 sac, now completely absorbed. YS, cavity of yolk-sac, continuous with the uterine 

 cavity owing to absorption of the lower wall of the yolk-sac. 



