200 Observations on the Beveloiwient 
to tli8 three before seen, a fourth branchial artery. These four, 
with the primitive arch, complete the normal number of five 
arterial arches. As the first branchial artery, the second arterial 
arch, increases in size, the first arch rapidly diminishes. The sub- 
sequent changes, noticed as long as they could be seen, were as 
follows : — The heart, by the diminished bulk of the yelk-bag, &c., 
becomes more central, the anterior chamber retains its old position, 
but the posterior, instead of being external, is now internal to the 
anterior. This change of position would appear to take place in 
some such way as this (Fig. 29) : — By the rapid growth and deve- 
lopment of structures near to the heart, closing in the praecordial 
area, the large veins become median to the axis of the body ; the 
posterior chamber into which they enter is drawn after them, 
bending, we may imagine, round an axis x, x, inwards, whilst the 
prominent wall of the anterior chamber sinks down from a to a^, 
so that the appearance of the heart as last seen was that of Fig. 28, 
where the arrows indicate the direction of the blood-current — a, the 
posterior, h, the anterior chamber. The course of the blood through 
the heart is seen to be remarkably serpentine. 
I am disposed to look upon these heart cavities as follows : — 
(Fig. 22) the space in which c is placed, as the auricle ; (Fig. 39) 
the space in which a is placed, as the pericardium ; and the posterior 
and anterior chambers, as the ventricle, and bulbus arteriosus. 
In the branchial system of vessels a most singular change takes 
place, whereby the direction of the blood-current in a portion of the 
main vessel is reversed. Before this change occurs, the course of 
the blood is as follows (Fig. 30) : — Starting from the heart it divides 
at gi, into h, the branchial arteries which enter the aortic circle, and 
(/, the primitive arterial trunk ; the latter ascends to i, and there 
gives off supplies to the mouth, eye, and head, subsequently form- 
ing part of the aortic circle, and running downwards. After a time, 
fourteenth or fifteenth day, a small vessel (?) forms, running from 
the first branchial artery (h) to the primitive trunk. This is the 
venous or refluent branch of the first branchial artery. This last- 
named artery increases in size, and in a corresponding degree the 
primitive trunk not only ceases to enlarge, but becomes less. At 
last a shght ridge appears between the first and second vascular 
arches (at g) ; this ridge grows forward so as to press upon the 
primitive trunk. By means of this pressure the artery gradually 
dwindles away until it is obliterated from g to i. The course of 
the blood now is that of the adult circulation. It passes from the 
heart to the first (second, third, and fourth) branchial arteries, and 
enters the aortic circle. Part goes downwards, part goes upwards 
in a direction diametrically opposed to the former course, running 
from m to where it gives off a, the carotid, to supply the head ; 
d, e, the orbits nasal, for the structures protecting the organs of 
