62 GENERAL EMBRYOLOGY. 
inferior vena cava. The mixed blood from the inferior vena cava passes through 
the right auricle, traverses the foramen ovale in the interauricular septum, and 
enters the left auricle; from the left auricle it is transferred to the left ventricle 
through the left auriculo-ventricular opening, and the left ventricle ejects it into 
the aorta. From the first part of the aorta some of the blood passes into the 
vessels which supply the head and neck and upper extremities, the remainder 
mixes with the blood conveyed to the aorta by the ductus arteriosus, and the 
blood, thus further mixed, is in part distributed to the walls of the thorax and 
abdomen and to the lower extremities, and in part passes to the placenta. 
Before birth, therefore, there is no pure arterial or fully oxygenated blood in 
the arteries of the foctus. The blood entering the heart by the superior vena cava 
is venous blood from the head, neck, upper extremities, and thorax; that entering 
by the inferior vena cava is mixed blood, consisting of venous blood from the lower 
part of the body and the lower extremities, and arterial blood from the placenta. 
The two streams do not mix in the right auricle, but the mixed or more arterial 
stream passes directly through the right into the left auricle, thence into the left 
ventricle, and from the left ventricle into the aorta or main systemic vessel, which 
conveys it to all parts of the body. The different parts of the body do not, however, 
receive equally oxygenated blood, for the venous stream which enters the right 
auricle by the superior vena cava, passes through that cavity into the right ventricle ; 
by the right ventricle it is forced into the pulmonary artery, from which some small 
part passes into the lungs, and so back to the left auricle by the pulmonary veins, 
but by far the greater part is carried by the ductus arteriosus to the aorta, which it 
enters beyond the origins of the vessels which supply the head, neck, and upper 
extremities ; therefore the blood in the lower part of the aorta, which is distributed 
to the abdomen, lower limbs, and placenta, is much more mixed or impure (less 
oxygenated) than that which is distributed to the head, neck, and upper extremities 
from the upper part of the aorta. 
SUMMARY OF THE EXTERNAL FEATURES OF THE HUMAN 
EMBRYO AT DIFFERENT PERIODS OF DEVELOPMENT. 
The first week.—The phenomena of fertilisation and segmentation have not been— 
observed in the human ovum, but there is no reason to believe that they differ in any 
essential respect from those met with in the ova of other mammals. — Fertilisation probably 
occurs in the upper part of the Fallopian tube, and segmentation is completed in the 
lower part of the same canal by the eighth or tenth day, when, presumably, the ovum 
becomes a morula, and passes, either as such or as a blastula, into the cavity of the— 
uterus. 
The second week.—At the twelfth day the ovum is embedded in the uterine wall ; 
it is a lenticular vesicle, 
which measures 5°5 mm. 
(4 of an inch) in length and 
3°3 mm. (4 of an inch) in 
breadth. Its upper and 
lower surfaces are smooth 
and convex, the latter 
being somewhat flatter 
than the former, and it is 
surrounded —equatorially 
Fic. 48. by a broad band of villi, 
A. Human embryo at the end of the 12th day of development; B. At the some of which are slightly 
AM 
end of the 13th day of development ; C. At the end of the 14th day of branched. The wall of 
development. (After His.) 
AM. Amnion ; AS. Allantoic stalk ; BS. Body-stalk ; CV. Chorionic villi on .. wae hs e 
asegment of the chorion; EK. Embryo; H. Head of embryo ; PR. Peri- OSS Mi hich project 
cardial region ; SS. Stomatodwal depression ; YS. Yolk sac. from it consist of ecto- 
dermal cells, and in the 
embryonic area, which is clearly marked on the upper surface, there is an inner layer 
of granular nucleated corpuscles. 
“By the end of the twelfth or the beginning of the thirteenth day the length of the 
the vesicle and the villous | 
ovum has increased to 6 mm. (4 inch), and its breadth to 4:5 mm. Linch). The embryonic — 
