REPRODUCTION. 



663 



The allantois is of especial interest and importance, as it is the 

 means by which the blood of the embryo is brought into relation 

 with the blood of the mother. As it develops, two arteries, the 

 hypogastrics, one from each internal iliac, pass out of the abdominal 

 cavity within the walls of the allantois, and follow it in its course 

 around the embryo. The ultimate branches of these arteries pene- 

 trate the villous processes which develop on the surface of the chorion 

 and which take part in the formation of the placenta. A single 

 large vein emerges from the placenta and returns the blood to the 

 embryo. In its course it winds around the arteries in a spiral 

 manner a number of times. These vessels the umbilical arteries 

 and vein are enclosed by the walls of the allantois and amnion, 

 and together constitute the umbilical cord which at the end of gesta- 

 tion is about 60 cm. in length. 



The Chorion. The cho- 

 rion, the external investment 

 of the embryo, is formed by 

 the fusion of the primitive egg 

 membrane the zona pellu- 

 cida the external layer of the 

 amnion, and the allantois. 

 Very early in development its 

 external surface becomes 

 covered with homogeneous, 

 granular, club-shaped proces- 

 ses, which by continued bud- 

 ding and growth, give to the 

 membrane a shaggy appear- 

 ance. At about the end of 

 the second month these pro- 

 cesses begin to atrophy and 

 disappear from the surface of the chorion, with the exception of that 

 portion which is in contact with the decidua serotina. At this point 

 the processes or villi continue to grow and develop, and insert them- 

 selves more deeply into the mucous membrane. Corresponding 

 processes from the mucous membrane insert themselves between 

 the villi of the chorion, which by their growth and fusion secure, 

 among other things, the retention of the embryo. 



The Nutrition of the Embryo. Coincidently with the develop- 

 ment of the amnion, allantois, and chorion, there arises within the 

 body of the embryo the early forms of many, if not all, of the future 

 viscera. The nutritive material required for their growth is partly 

 contained within the umbilical vesicle lying without the body cavity. 

 That this material may be utilized, blood-vessels emerge from the 

 body and ramify within the walls of the vesicle. The capillaries to 



FIG. 330. HUMAN EMBRYO AND ITS EN- 

 VELOPES AT THE END OF THE THIRD 



MONTH. 



