DEVELOPMENT OF THE VASCULAR SYSTEM. 105 
With the development of the placenta the allantoic circula- 
tion renders the vitelline- subordinate, the vitelline and the larger 
mesenteric vein forming the portal. The portal vein at a later 
period joins one of the venee advehentes of the allantoic vein. 
At first the vena cava inferior and the ductus venosus enter 
the heart as a common trunk. The ductus venosus Arantii 
becomes a small branch of the vena cava. 
The allantoic vein is finally represented in its degenerated 
form as a solid cord (rownd ligament), the entire venous sup- 
ply of the liver being derived from the portal vein. 
The development of the heart has already been traced in the 
fowl up to a certain point. In the mammal its origin and early 
progress are similar, and its further history may be gathered 
from the following series of representations. 
In the fowl the heart shows the commencement of a division 
into a right and left half on the third day, and about the 
fourth week in man, from which fact alone some idea may be 
gained as to the relative rate of development. The division 
Fig. 121. ° 
Fie. 121.—Development of the heart in the human embryo, from the fourth to the sixth week. 
A. Embryo of four weeks (Kélliker, after Coste). B, anterior, C, posterior views of the 
heart of an embryo of six weeks (K6lliker, after Ecker). a, upper limit of buccal cavity ; 
¢e, buccal cavity ; b, lies between the ventral ends of second and third branchial arches ; 
d, buds of upper limbs ; J, liver ; f, intestine ; 1, superior vena cava ; 1’, left superior vena 
cava; 1’, opening of inferior vena cava; 2, 2’, right and left auricles ; 3, 3’, right and left 
ventricles ; 4, aortic bulb. 
Fic. 122.—Human embryo of about three weeks (Allen Thomson). uv, yelk-sac; al, allantois; 
am, amnion ; ae, anterior extremity ; pe, posterior extremity. 
is effected by the outgrowth of a septum from the ventral wall, 
which rapidly reaches the dorsal side, when the double ven- 
tricle thus formed communicates by a right and left auriculo- 
ventricular opening with the large and as yet undivided auricle. 
