50 | GENERAL EMBRYOLOGY. 
together, and the whole is surrounded by the expanding amnion. In this way the 
umbilical cord is formed. It includes therefore the allantoic stalk and its blood- 
vessels, together with the remains of the yolk-sac and its stalk, the vitello-intestinal 
duct, all of which are invested and bound together by the amnion. 
The mesodermal core of the cord forms a fibro-mucoid tissue known as 
“Wharton’s jelly,” which consists of stellate and irregular cells embedded in a 
gelatinous matrix. The blood-vessels of the cord are situated in the core, and 
include two allantoic or umbilical arteries which run spirally round a single 
umbilical vein. The terminal portion of the allantoic diverticulum projects into 
the embryonic end of the cord, and at first a loop of intestine protrudes into it for 
a short distance; the gut, however, soon recedes into the abdominal cavity. 
The umbilical cord, which extends from the umbilical orifice to the centre of the 
placenta, is at first short and straight. As the amnion expands the length of the 
umbilical cord increases until, at “the time of birth it measures, On an average, 
about 20 inches. This increase in the length of the cord allows the foetus to float 
freely in the amniotic fluid, and prevents traction on the placenta. 
After the middle of the second month the umbilical cord is twisted spirally, 
usually from right to left. It is suggested that this is due either to the great 
elongation of the allantoic arteries or to muscular movements of the foetus, and it 
involves a rotation of the foetus in the amniotic fluid. 
The Chorion.—The chorionic area, by far the largest of the areas into which 
the blastoderm is divisible, lies external to the amniotic area. In most mammals 
it consists at first of ectoderm and entoderm, but after the extension and cleavage 
of the mesoderm has taken place, it is formed by ectoderm and somatic mesoderm. 
In man, however, it consists in the earliest stages of ectoderm alone, but on the 
formation and extension of the mesoderm it also acquires an inner layer of somatic 
mesoderm. In all cases, therefore, it eventually consists of the same two layers. 
The ectoderm of the chorionic area which unmediately surrounds the ammiotie 
area thickens to form the annular placental area, and in this way the chorionic area 
becomes divisible into placental and non-placental regions. 
When the blastodermic vesicle enters the uterus numerous ectodermal villous 
processes grow from the surface of the chorionic area, both in its placental and non- 
placental parts, and attach themselves to the uterine mucous membrane. As 
already pointed out in the description of the formation of the amnion, the embryonic 
and amniotic areas do not become attached to the uterus, but remain free from it, 
whilst by the approximation and fusion of its inner margins, the rapidly growing 
ring-like placental area is converted into a disc which intervenes between the 
amnion and the uterme wall. ' 
The chorionic area after the separation of the amnion is known as the chorion 
or chorionic membrane. 
The chorion forms the outer wall of a vesicle, the chorionic vesicle, which is the 
modified remains of the blastodermic vesicle and which contains the embryo, the 
yolk-sae, the amnion, and the allantois. It consists of an outer layer of ectoderm 
and an inner layer of somatic mesoderm. 
The cavity of the chorion is the extra-embryonic portion of the coelom. For a 
time it remains distinct, and is traversed by the allantoic stalk which unites the 
embryo to the inner or mesodermal layer of the placental area. The cavity is 
ultimately obliterated by the growth of the amnion, the latter sac expanding 
rapidly till its outer surface is in contact and intimately blended with the inner 
surface of the chorion. 
The Chorionic Villi—The villous processes which begin to grow from the 
surface of the chorionic area before it is separated from the amnion continue to 
develop after the separation of the two membranes is completed. They penetrate 
the surrounding uterine tissues. At first each consists of ectoderm only, but a 
core of vascular mesoderm is soon acquired. The villi increase in size and in com- 
plexity also, but ultimately only those in the placental area persist and continue 
to grow; the remainder atrophy and disappear. 
“Thus the placental region of the chorion eventually constitutes the main bond of 
union between the ovum and the mother, and it forms the foetal part of the placenta. 
