THE EVIDENCE OF THE RESPIRATORY APPARATUS 179 
A priori, such a derivation seems highly improbable; and yet it 
is precisely the manner in which embryology teaches us that the 
heart and ventral aorta of the vertebrate have arisen. 
THE ORIGIN OF THE INVERTEBRATE HEART AND THE ORIGIN OF THE 
VERTEBRATE HEART, 
Not only does the vertebrate heart differ from that of the inverte- 
brate, in that it is branchial while the latter is systemic, but also it 
is unique in its mode of formation in the embryo. In the Appen- 
diculata the heart is formed as a single organ in the mid-dorsal line 
by the growth of the two lateral plates of mesoblast dorsalwards, 
the heart being formed where they meet. In Mammalia and Aves, 
the heart and ventral aorta commence as a pair of longitudinal veins, 
one on each side of the commencing notochord. 
Tf the embryo be removed from the yolk, the surface of the embryo 
covering these two venous trunks can be spoken of as the ventral 
surface of the embryo at that stage, and indeed we find that in the 
present day there is an increasing tendency to speak of this surface 
as the ventral surface of the embryo. Thus, Mitsukuri, in his studies 
of chelonian embryos, lays great stress on the importance of surface 
views and when the embryo has been removed from the yolk, 
figures and speaks of its ventral surface. So, also, Locy and Neal 
find that the best method of seeing the early segments of the embryo 
is to remove the embryo from the yolk, and examine what they speak 
of as a ventral view. At the period, then, before the formation of the 
throat, we may say that on the ventral surface of the embryo a pair 
of longitudinal venous sinuses are found, one on each side of the mid- 
ventral line, which are in the same position with respect to the mid- 
axis of the embryo as are the longitudinal venous sinuses in Limulus. 
The next step is the formation of the throat by the extension of 
the layers of the embryo laterally to meet in the mid-line and so 
form the pharynx, with the consequence that a new ventral surface is 
formed ; these two veins, as is well known, travel round also, and, 
meeting together in the new mid-ventral line, form the subintestinal 
vein, the heart, and the ventral aorta. 
What is true of Maimalia and Aves, has been shown by P. Mayer 
to be true universally among vertebrates, so that in all cases the heart 
and ventral aorta have arisen by the coalescence in the new mid-ventral 
a 
