142 
MR. MARSHALL ON THE DEVELOPMENT 
length (Plate III. figs. 1, 2), the cross branch {d) was already formed. Owing to the 
width of the human thorax, the jugular veins do not approach each other closely at 
the root of the neck, as in the Sheep ; the cross branch continues of a much greater 
length, and, at first horizontal in its direction, only assumes after a considerable 
time, the somewhat oblique position which it permanently holds as the left innomi- 
nate vein. Rathke describes it as formed opposite the point of junction of the 
jugular and subclavian veins; but in the embryos dissected by myself, it is a little 
hwer down than that point and immediately above the commencing pericardium. 
b. Occlusion of 'part of the Left Primitive V zins. 
It is not until after the transverse connecting branch is already nearly as large as 
either jugular vein, that the venous channels destined to be occluded begin to shrink. 
Previously to the commencement of this stage of the metamorphosis, the cardinal 
veins (figs. 1, 2, &, V) have much diminished in size owing to the wasting of the Wolf- 
fian bodies (5) ; but the two halves of the venous system are still quite symmetrical, 
except that the arch of the right cardinal vein is higher up than the left, in accord- 
ance with the greater size and relative altitude of the corresponding lung, as already 
pointed out in the Sheep. From opposite the cross branch, the two jugular veins 
descend behind the auricular appendices outside the pericardium, to become con- 
tinuous with the canals of Cuvier (c, d), which, having received the cardinal veins, 
immediately enter the pericardium, and bend inwards beneath the auricular portion 
of the heart, into which they open, one on each side of and somewhat before the in- 
ferior cava. 
In addition to a part of the left primitive jugular vein, lying outside the pericar- 
dium below the cross branch in the neck, it will be found that in the human subject, 
in which there is no left azygos as in the Sheep, the left cardinal vein shrinks into 
an insignificant branch, and entirely disappears as a distinct trunk; whilst the greater 
part of the left canal of Cuvier, placed within the pericardium, is also closed ; its last 
part, however, situated on the back of the left auricle, remaining permanently open. 
This pervious portion of the Cuvierian canal eventually forms the coronary sinus. 
The first step towards this occlusion consists in a gradual shrinking of the left 
primitive venous channel from the left end of the cross branch down to the back of 
the heart, as shown in an embryo of 1 inch and 2 %ths in length (Plate III. fig. 3). The 
upper portion of the vein lies upon the aorta and ductus arteriosus, beneath the left 
pleura ; whilst the lower portion, within the pericardium, crosses in front of the vessels 
of the left lung, lodged in a dupUcature of the serous membrane, reminding one of 
the fold in which the left azygos vein is contained in the adult Sheep. The lower end 
of the vessel, placed on the back of the left auricle, is more dilated than the rest. In a 
more advanced embryo, 1 inch and -|^ths long, the closure was complete, and had pro- 
bably been accomplished a considerable time (fig. 4). The place of the primitive vein 
is now seen to be occupied by a fine cord (c"), which may be followed from the under 
