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 
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 
by means of which we 
searcely find a trace in Oviparous 
animals ; yet the ova of both are 
iginally 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 Fig. 326.—Emeryo oF THE Fowt, 
and of the Nervous centres may, sith enbiigaeadionsce retin 
however, be noticed, as character- ; 4, folds of germinal membrane, 
istic examples of the mode in  teral folds; ¢, e, rudiments of 
which the evolution of the several pti ganglia and cerebrum; f, 
of the body takes place — ~_ venous trunk, forming atrium of 
The Heart, in Man and other © oa’ 2.20mi?) 3.$ prem 
Mammals, as in the Bird (§ 758), _ vascular area; &, &, dorsal lami- 
is at first a simple tube, resembling ache’ 
the pulsatile trunk that remains as 
the sole organ of impulsion im 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 
