DEVELOPMENT OF CIRCULATING APPARATUS. 589 
walls of both, the materials requisite for its growth ; but there 
is no direct communication between the two. The same means 
serve for the aeration of the blood of the embryo; for this, 
being brought from its body in the venous condition, is exposed 
to the influence of the arterial blood of the parent, through 
the thin walls of its vessels,—just as the venous blood of aquatic 
animals is aerated in their gill-tufts,—and passes back to the 
embryo in the arterial condition, having imparted its carbonic 
acid to the blood of the parent, 
and received from it oxygen.— 
Thus all but the very early stages 
of development are performed in 
Mammals, by means of which we 
scarcely find a trace in Oviparous 
animals ; yet the ova of both are 
originally formed on the same plan, 
and the first changes which they 
undergo are exactly analogous. 
762. It would not be consistent 
either with the design or with the 
limits of this work, to enter in 
much detail into the considera¬ 
tion of the processes of develop¬ 
ment, although they present many 
points of the highest interest. 
The general history of the evolu¬ 
tion of the Circulating apparatus 
and of the Nervous centres may, 
however, be noticed, as character¬ 
istic examples of the mode in 
which the evolution of the several 
organs of the body takes place.— 
The Heart, in Man and other 
Mammals, as in the Bird (§ 758), 
is at first a simple tube, resembling 
the pulsatile trunk that remains as 
the sole organ of impulsion in the lowest forms of circulating 
apparatus. After a time this tube is doubled upon itself, and 
two cavities are formed, an auricle and a ventricle ; in this con¬ 
dition, it strongly resembles the heart of the Fish (§ 286). The 
circulation too is, at an early period, that of the Fish; for the 
Fig. 326.— Embryo of the Fowl, 
from the Ovum shown in fig. 
318, greatly enlarged: 
a, b, folds of germinal membrane* 
enveloping head and tail; c, la¬ 
teral folds; d, e, rudiments of 
optic ganglia and cerebrum; /, 
heart; g, dilated termination of 
venous trunk, forming atrium of 
heart; h, aorta; 1, 2, 3, 4, bran¬ 
chial arches; i, i, vessels of 
vascular area; k, Jc, dorsal lami¬ 
nae; l, l, rudiments of vertebral 
arches. 
