em; which blood is again taken up by the maternal veins, and is
thus perpetually changed. While the terminations of the placental arteries
and veins are spread in fine reticulation on the sides of these cells. And
thus, as the growing fetus requires greater oxygenation, an apparatus is
produced resembling exactly the air-cells of the lungs.

In cows, and other ruminating animals, the internal surface of the uterus
is unequal like hollow cups, which have been called cotyledons; and into
these cavities the prominencies of the numerous placentas, with which the
fetus of those animals is furnished, are inserted, and strictly adhere;
though they may be extracted without effusion of blood. These inequalities
of the uterus, and the numerous placentas in consequence, seem to be
designed for the purpose of expanding a greater surface for the
terminations of the placental vessels for the purpose of receiving
oxygenation from the uterine ones; as the progeny of this class of animals
are more completely formed before their nativity, than that of the
carnivorous classes, and must thence in the latter weeks of pregnancy
require greater oxygenation. Thus calves and lambs can walk about in a few
minutes after their birth; while puppies and kittens remain many days
without opening their eyes. And though on the separation of the cotyledons
of ruminating animals no blood is effused, yet this is owing clearly to the
greater power of contraction of their uterine lacunae or alveoli. See
Medical Essays, Vol. V. page 144. And from the same cause they are not
liable to a sanguiferous menstruation.

The necessity of the oxygenation of the blood in the fetus is farther
illustrated by the analogy of the chick in the egg; which appears to have
its blood oxygenated at the extremities of the vessels surrounding the
yolk; which are spread on the air-bag at the broad end of the egg, and may
absorb oxygene through that moist membrane from the air confined behind it;
and which is shewn by experiments in the exhausted receiver to be
changeable though the shell.

This analogy may even be extended to the growing seeds of vegetables; which
were shewn by Mr. Scheele to require a renovation of the air over the
water, in which they were confined. Many vegetable seeds are surrounded
with air in their pods or receptacles, as peas, the fruit of staphylea, and
lichnis vesicaria; but it is probable, that those seeds, after they are
shed, as well as the spawn of fis