GENERAL EMBRYOLOGY 1627 



The Yolk-Sac and Vitelline Duct. 



The yolk-sac is the ventral compartment of the cav-ity of the 

 original blastodermic vesicle. As the formation of the amnion is 

 being completed, an infolding of splanchnoplenre takes place. The 

 blastodermic vesicle is thereby gradually constricted, and its cavity 

 is divided, at first incompletely, into two compartments — namely, 

 dorsal and ventral. The dorsal compartment, which is the smaller 

 of the two, gives rise to the mid-gut (primitive gut), and is situated 

 within the body of the embryo. It is at first a groove, kno\\-n as 

 the intestinal groove, but it is subsequently converted into a tube. 

 The ventral compartment which is large and capacious, forms the 

 yolk-sac or umbilical vesicle, and is extra-embryonic. 



The yolk-sac is at first in wide communication with the centre of 

 the primitive mid-gut by a passage called the vitelline or vitello- 

 intestinal duct (yolk-stalk) which passes through the umbilical 

 opening, and is connected with the ventral aspect of the mid-gut. 

 The wall of the yolk-sac and vitelline duct is continuous with that 

 of the mid-gut, both consisting entirely of splanchnoplenre — ^that 

 is to say, entoderm and splanchnic mesoderm, the entoderm forming 

 the lining membrane. 



In the human embryo the yolk-sac contains fluid. Its wall 

 presents a well-marked vascular area, with which the primitive 

 bloodvessels are associated. These vessels are called the vitelline 

 or omphalo-mesenteric vessels and are four in number, two arteries 

 and two veins, which will be again referred to. 



The yolk-sac undergoes atrophy, but a remnant of it persists in 

 the umbilical cord. The \'itelline duct also undergoes atrophy, and 

 its lumen disappears. The part of the duct, however, which is 

 connected with the mid-gut sometimes remains persistent, and so 

 gives rise to an intestinal protrusion, known as Meckel's diverticulum. 



The Chorion. 



The chorion is the outer of the two foetal deciduous membranes, 

 the one internal to it being the amnion. It is developed from the 

 outer layers of the amniotic folds, and, like the amnion, is composed 

 of somatopleure. Its external surface consists of extra-embryonic 

 ectoderm, and the internal surface is formed by somatic mesoderm. 



The chorion is situated external to the amnion, and the somatic 

 mesoderm of its inner surface faces the somatic mesoderm which 

 forms the outer surface of the amnion. The interval between the 

 chorion and the amnion is known as the chorionic cavity, and the 

 embryo is now surrounded by two membranes, the inner of which 

 is the amnion. 



The chorionic cavity represents the extra-embryonic portion of 

 the coelom or body-ca\nty, and it contains (i) the embryo, enclosed 

 within the amnion; and (2) the yolk-sac or umbilical vesicle. It is 



