DECIDUAL MEMBRANES. 



301 



cho 



well known, but the youngest which have been obtained are more advanced 

 than B ; therefore the stages A and B are hypothetical. 



In further development the amniotic cavity enlarges so that the am- 

 nion is in contact with the inner surface of the chorion (Fig. 344). The 

 yolk sac and its attenuate stalk are brought close to "the allantois. The 

 mesenchymal tissue surrounding the yolk stalk and allantois and covered 

 by a layer of ectoderm, forms the umbilical cord. At first it contains an 

 extension of the body cavity around the yolk stalk but later this is obliter- 

 ated by adhesions. The ectoderm of the cord is continuous distally 

 with that of the amnion and proximally with the epidermis of the embryo. 

 There is an abrupt transition from the skin with its capillaries to the non- 

 vascular covering of the cord, which at birth is about 8 mm. from the 

 abdominal wall. 



Only one side of the chorionic 

 vesicle becomes implanted upon the 

 uterine mucosa. The villi on that side 

 of the vesicle proliferate and constitute 

 the chorion frondosum. Elsewhere 

 the villi become scattered and low, 

 finally disappearing; the resulting 

 smooth part of the chorion is called 

 the chorion laeve. 



The appearance of a human em- 

 bryo at a stage intermediate between 

 the last two diagrams considered, is 

 shown in Fig. 345. The greater part 

 of the villous chorion has been cut 



away together with half of the thin smooth amnion, thus exposing the 

 embryo with its umbilical cord and yolk sac. 



Relation between the membranes and the uterus. That portion of the 

 uterine mucosa against which the chorionic vesicle rests and into which 

 its villi proliferate, is called the decidua basalis [serotina]. A portion which 

 grows over the vesicle completely enclosing it is the decidua capsularis 

 [reflexa]. The remainder of the mucosa is the decidua vera (Fig. 346). 

 As the embryo increases in size so as to fill and distend the uterine cavity, 

 the decidua capsularis becomes thin, degenerates, and is resorbed so that 

 in the last half of pregnancy the chorion laeve rests directly upon the 

 decidua vera (Fig. 346, B). The chorion frondosum together with the 

 inseparable part of the decidua basalis into which its villi have grown, 

 form the placenta. Thus the placenta consists of a uterine and a fetal 

 portion. It is a discoid mass of vascular tissue which at birth is about 



FIG. 344. DIAGRAM OF THE FORMATION- OF 

 THE UMBILICAL CORD, LETTERED AS ix 

 FIG. 343- 



