521 
of Edinburgh , Session 1868-69. 
and Dumas, as well as Wagner and Allen Thomson, describe the 
first heart as a conical sac, having its apex anterior beneath the head 
of the embryo, with two inferent veins entering from above, and only 
one efferent tube, the ductus venosus. Wagner mentions a certain 
amount of motion among the blood-discs, previous to the contraction 
of the conical heart, having only a single cavity. It is at first 
transparent and membranous, but contractile, and filled with a 
yellowish or pale orange fluid, the plasma or liquor sanguinis. As 
soon as the punctum saliens in the base of the heart starts its 
contraction, its fluid is forced down the ductus venosus to the 
oxidising area of the chorion, where it is fully arterialised, and 
follows the course, already shortly described, to the aorta, its ter- 
minal branches, forming the aortic bulb, terminate in the carotid and 
subclavian arteries to the head, upper trunk, and extremities. From 
thence the blood, reduced to a venous condition, is returned by the 
jugular and subclavian veins forming the superior venae cavae, 
which flow into the single cavity of the primary heart. The thin 
condition of its transparent walls might lead us to view it as 
analogous to the lymph heart of the frog and tail of the eel, so 
well described by Mr Wharton Jones, F.R.S., in a communica- 
tion published in the last volume of the Philosophical Trans- 
actions, forcing on a column of dark venous blood discs. 
II. Soon after the blood has circulated once or twice, there is 
a ventricle added to the base of the auricle, which is then perforated 
by the auriculo-ventricular orifice. Still there is only the same 
outlet for the venous blood from the heart, which is, during foetal 
non-breathing existence, entirely connected alone with the venous 
circulation. 
III. The next change in the structure of the heart is the dividing 
the ventricle by a muscular septum, which, till it is complete, 
allows the venous blood to enter freely into both ventricles (a 
system which continues in some reptiles), from which it is instantly 
returned to the auricle by their systole, and forced down the 
ductus venosus to be oxidised and returned as arterial by the umbi- 
lical arteries into the iliacs, and upwards by the aortae, as already 
described. 
IV. After the completion of the ventricular septum there com- 
mences a septum in the auricle, never completed for at least a year 
