320 HUMAN UTERUS AND FCETAL APPENDAGES. 



crucial incision on the anterior side ; its walls will be found about an inch or more 

 in thickness; it contains a grayish-red bag (decidua reflexa), which nearly fills 

 the cavity of the uterus and encloses the embryo, so that upon opening the womb 

 we do not encounter the foetus directly. The inner bag has a smooth surface, 

 but shows a few small pores; it is without blood-vessels and is attached to the 

 dorsal wall of the uterus. The inner surface of the uterus shows a rich network 

 of blood-vessels, many of which are large, irregular sinuses. The uterine walls 

 consist of an outer muscular layer, and an inner decidual layer, which takes up 

 nearly half the thickness of the wall, and is known as the decidua vera. Com- 

 parison with the seventh month uterus shows that the proportion of the layers 

 changes, because during gestation the muscular layer increases and the decidual 

 layer diminishes in 'thickness. The inner bag, when opened, shows the large cav- 

 ity in which the embryo lies, floating in amniotic fluid. The bag is formed by 

 three very distinct membranes, of which the outermost, the decidua reflexa, is 

 opaque and the thickest ; the two inner ones are thin and transparent ; the inner- 

 most is the delicate amnion; the middle membrane is the chorion, and is quite 

 distinct from both the amnion and reflexa ; with the latter it is connected by a 

 number of small branching villi scattered at some distance from one another over 

 the surface; the villi adhere firmly to the reflexa by their tips. The embryo 

 (Fig. 94) resembles a child in its general appearance; the length of the head 

 and rump together is about 8 cm., and the head is approximately of equal bulk 

 to the rump. The umbilical cord is from 5 to 7 mm. in diameter and usually about 

 1 2 cm. long. From its distal end the blood-vessels spread out over the placental 

 area, and around the edge of the area rises the decidua reflexa, which does not 

 extend on to the placenta. Floating in the amniotic fluid is a pear-shaped 

 vesicle, the yolk-sac, which is about 7 mm. in diameter; it has a fine network of 

 blood-vessels upon its surface, and is connected at its pointed end with a long 

 slender pedicle, the yolk-stalk, which runs to the placental end of the umbilical 

 cord, there enters the cord itself, and runs through its entire length to its attach- 

 ment to one of the coils of the intestine of the embryo.* Over the whole of the 

 placental area the chorion gives off large villous trunks, each of which has numer- 

 ous branches, with ramifications of the foetal vessels; the villi fill a space about 

 one centimeter wide between the membrane of the chorion frondosum and the 

 surface of the uterine decidua serotina, to which the tips of some of the villi are 

 attached. With care the villi may be separated from the decidua, which is seen, 

 when it is thus uncovered, to be cavernous ; the caverns are rounded in form and 



* At this stage a large part of the yolk-stalk within the umbilical cord has degenerated and usually disap- 

 peared by resorption. 



