1x.] THE BRAIN. 281


Up to this period the walls of the somatic stalk
have remained thin and flaccid, almost membranous in
fact, the heart appearing to hang loosely out of the
body of the embryo. About this time however the
stalk, especially in front, rapidly narrows and its meso-
blast becomes thickened. In this way the heart and
the other thoracic viscera are enclosed by definite firm
chest walls, along the sides of which the ribs grow
forwards and in front of which the cartilaginous rudi-
ments of the sternum appear.
The abdominal walls are also being formed, but not
to the same extent, and the stalk of the allantois still
passes out from the peritoneal cavity between the
somatic and the splanchnic stalks.
In the brain one of the most marked features is the
growth of the cerebral hemispheres. The median division
between these has in front increased in depth, so that
the lateral ventricles are continued forwards as two
divergent horns, while backwards they are also con-
tinued as similar divergent horns separated from one
another by the vesicle of the third ventricle.
We propose to treat more fully of the development of the
brain in the second part of this work, the importance of the
mammalian brain rendering it undesirable to go too much into
the details of the brain of the bird.
All the visceral clefts are closed by the seventh day.
It will be remembered that the inner part of the first
cleft persists as the Eustachian tube (p. 166).
The structures which surround the mouth are be-
ginning to become avian in form, though the features
are as yet not very distinctly marked.
282 THE SIXTH DAY. [CHAP.
