the outgrowing allantois,
pushing in front of it the serous membrane, is closely applied to the
lining of the mother's uterus. The maternal uterus and the embryonic
allantois send out finger-like processes into each other which
interlock, and the tissue between the abundant bloodvessels in them
thins down to such an extent that nutritive material, peptones and
carbohydrates, and oxygen also, diffuse freely through it from
mother to foetus,* and carbon dioxide, water, and urea from the
foetus to the mother. The structure thus formed by the union of the
wall of the maternal uterus, allantois, and the intermediate structures
is called the placenta. Through its intermediation, the young rabbit
becomes, as it were, rooted and parasitic on the mother, and utilizes
her organs for its own alimentation, respiration, and excretion. It
gives off CO2, H2O, and urea, by the placenta, and it receives O and
elaborated food material through the same organ. This is the better
method that has superseded the yolk.

* The embryo.

Section 39. In its later development, the general facts already
enunciated with regard to the organs of frog and fowl hold, and where
frog and fowl are stated to differ, the rabbit follows the fowl. In the
circulation the left fourth vascular arch (second branchial) gives rise to
the aortic arch; in the right the corresponding arch disappears, except
so much of it as remains as the innominate artery. The azygos vein
(Chapter 3) -is a vestige of- [is derived from] the right posterior
cardinal sinus. Both pulmonary arteries in the rabbit are derived from
the left sixth vascular arch (= fourth branchial). Compare Section 32. The
allantois altogether disappears in the adult fowl; in the adult mammal
a portion of its hollow stalk remains as the urinary bladder, and the
point where it left the body is marked by the umbilicus or navel. The
umbilical arteries become the small hypogastric arteries on either side
of the urinary bladder. There is no trace of a pronephros at all in the
rabbit.

Section 40. We may note here the development of the eye. This is
shown in Figure 4, Sheet 24. A hollow cup-shaped vesicle from the
brain grows out towards an at first hollow cellular ingrowth from the
epidermis. The cavity within the wall of the cup derived from the brain
is obliterated, [and the stalk withers,] the cup becomes the retina, and
-its stalk- [thence fibres grow back to the brain to form] the optic
nerve. The cell