107). They arise at a time when the lateral folds
which form the ventral wall of the throat are only just
becoming visible. Hach half of the heart originates in
the same way as in the chick; and the layer of the
splanchnic mesoblast, which forms the muscular wall for
each part (ahh), has at first the form of a half tube open
below to the hypoblast.
On the formation of the lateral folds of the splanchnic
walls, the two halves of the heart become carried inwards


XIL] ARTERIAL SYSTEM. 407


and downwards, and eventually meet on the ventral
side of the throat. For a short time they here remain
distinct, but soon coalesce into a single tube.
In Birds, it will be remembered, the heart at first has the
form of two tubes, which however are in contact in front. It
arises at a time when the formation of the throat is very much
more advanced than in Mammalia; when in fact the ventral
wall of the throat is established as far back as the front end of
the heart.
In the lower types the heart does not appear till the ventral
wall of the throat is completely established, and it has from the
first the form of a single tube.
It is therefore probable that the formation of the heart as two
cavities is a secondary mode of development, which has been
brought about by variations in the period of the closing in of the
wall of the throat.
The later development of the heart is in the main similar to
that of the chick (p. 256 et seq.).
