The young of quadrupeds when they are near their full time
contain excrements, both liquid and in the form of solid lumps, the
latter in the lower part of the bowel and the urine in the bladder.
In those animals that have cotyledons in the womb the cotyledons
grow less as the embryo grows bigger, and at length they disappear
altogether. The navel-string is a sheath wrapped about blood-vessels
which have their origin in the womb, from the cotyledons in those
animals which possess them and from a blood-vessel in those which do
not. In the larger animals, such as the embryos of oxen, the vessels
are four in number, and in smaller animals two; in the very little
ones, such as fowls, one vessel only.
Of the four vessels that run into the embryo, two pass through
the liver where the so-called gates or 'portae' are, running in the
direction of the great vein, and the other two run in the direction of
the aorta towards the point where it divides and becomes two vessels
instead of one. Around each pair of blood-vessels are membranes, and
surrounding these membranes is the navel-string itself, after the
manner of a sheath. And as the embryo grows, the veins themselves tend
more and more to dwindle in size.
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