DECIDUAL MEMBRANES 



369 



given rise to the fore-gut and hind-gut, and the allantois now appears as 

 an appendage of the latter. 



In Fig. 369, B, the embryo is represented as rotated so that its head is 

 downward and its ventral side toward the left. It is now connected with 

 the membranes by an umbilical cord, the composition of which may be seen 

 by comparing A and B. Its principal constituent is the elongated body 

 stalk, containing the allantois and covered above and on the sides with 

 adherent amnion. Below, the amnion also forms the covering of the cord, 

 but here it is separated from the body stalk by an extension of the body 

 cavity. The yolk stalk passes from the primary loop of intestine through 

 the cavity of the umbilical cord to the yolk-sac, in which it terminates. 



cho 



cho. 



A B 



FIG. 369. DIAGRAMS ILLUSTRATING THE DEVELOPMENT OF THE EMBRYONIC MEMBRANES AND THE FOR 



MATION OF THE UMBILICAL CORD.' 

 al., Allantois; am., amnion; am. c., amniotic cavity; cho., chorion; coe., ccelom; y. s., yolk-sac. 



This sac is now lodged in its permanent position between the amnion and 

 chorion. Ultimately the parts of the allantois, yolk stalk and body 

 cavity within the cord are obliterated. 



The appearance of a human embryo at a stage intermediate between 

 those shown in Fig. 369 is reproduced in Fig. 370. An irregular piece cut 

 out from the chorionic vesicle forms the background of the picture. 

 Around the cut edges of this piece the shaggy chorionic villi are seen, 

 directed toward the wall of the uterus. At the top of the figure is the 

 spherical yolk-sac lodged between chorion and amnion, between which 

 the yolk stalk passes to the distal end of the umbilical cord, which it enters. 

 The amnion is a membranous sac completely enclosing the embryo; in 

 the figure, half of it has been cut away to show the embryo within. The 

 skin of the embryo is continuous with the covering of the umbilical cord, 

 and distally this covering is reflected and becomes continuous with the 

 amnion. 



In later stages the umbilical cord is greatly elongated. It contains 

 the umbilical vessels which pass between the embryo and the chorion, 

 24 



