preading over the embryo it
splits into two layers, the outer of which is known as the
_somatopleure_ and lines the parietal or ectodermal wall, while the
inner lines the entoderm and is called the _splanchnopleure_; between
the two is the coelom. The pericardial area is early differentiated from
the rest of the coelom and at first lies in front of the neural and
bucco-pharyngeal area; here the mesoderm stretches right across the
mid-line, which it does not in front and behind. As the head fold of the
embryo is formed the pericardium is gradually turned right over, so that
the dorsal side becomes the ventral and the anterior limit the
posterior; this will be evident on referring to the two accompanying
diagrams.

[Illustration: After Young and Robinson, Cunningham's _Text-Book of
Anatomy_.

FIG. 4.--Diagram of a Developing Ovum, seen in Longitudinal Section.

_f_, Spinal cord. _i_, Brain.
_g_, Notochord. _k_, Extra embryonic coelom.
_h_, Dorsal wall of alimentary canal. Other numbers as in fig. 3.]

The two primitive aortae lie at first in the ventral wall of the
pericardium, but with the folding over they come to lie in the dorsal
wall and gradually bulge into the cavity as they coalesce to form the
heart, so that the heart drops into the dorsal side of the pericardium
and draws down a fold of the membrane called the _dorsal mesocardium_.
In mammals A. Robinson (_Jour. Anat. and Phys._, xxxvii. 1) has shown
that no ventral mesocardium exists, though in more lowly vertebrates it
is present. Laterally the pericardial cavity communicates with the
general cavity of the coelom, but with the growth of the Cuvierian ducts
(see development of veins) these communications disappear. Originally
the mesocardium runs the whole length of the pericardium from before
backward, but later on the middle part becomes obliterated, and so the
two separate reflections from the parietal to the visceral layer,
already noticed, are accounted for.

Just behind the pericardium and in front of the umbilicus, which at
first are close together, the mesoderm forms a mass which is called the
_septum transversum_, and into this the developing lungs push bag-like
protrusions of the coelom, consisting of visceral and parietal layers,
and these eventually lose their connexion with the rest of the coelom,
as the diaphragm develops, and become the pleural cavities. After the
pericardium and pleurae