168 
MR. MARSHALL ON THE DEVELOPMENT 
cross branch is entirely occluded, and also the succeeding part of the left Cuvierian 
canal, the lower end of which, however, remains pervious as a little conical pouch (o' ) 
on the back of the left auricle. A fine cord (c") occupies the place of the occluded 
vessel. The left cardinal vein has disappeared. The cross branch, now oblique in 
direction, forms the left innominate vein (c?), which ends in the vena cava superior (A). 
Fig. 5. Embryo, l^ths of an inch long from vertex to coccyx, and fig. 6, embryo, 
3 inches from vertex to coccyx. In these embryos, a portion of the left primitive 
jugular vein immediately below the cross branch remains open, though reduced in 
size, and forms the trunk of the left superior intercostal vein (i ) ; the succeeding 
portion is occluded and forms a fine semitransparent cord, which is continuous 
through the pericardium, with the occluded part of the left canal of Cuvier (c"). 
This latter lies in a fold of the serous membrane in front of the left pulmonary 
vessels, and descends on the back of the left auricle to the conical pouch (c') above 
mentioned. The left cardinal vein (A') in fig. 6, relatively very much diminished in 
size, appears to form a branch of the left superior intercostal vein. 
PLATE IV. 
Several views of the hearts of human embryos to show the gradual alterations in 
the size, length and direction of the two canals of Cuvier, and the particular meta- 
morphosis of the left canal. Figs. 1 to 6 and figs. 8, 10 and 1 1 are views of the under 
surface of the organ. Figs. 7 and 9 are representations of its left side. All the figures, 
excepting fig. 11, are shown magnified two diameters. 
The letters of reference employed in the several figures are as follows, a. Right 
and a' left jugular, c. Right, and c', left canal of Cuvier, c". Occluded portion of 
this canal, d. Cross branch in the neck, or left innominate vein. e. Vena cava in- 
ferior. fv I o s. Parts occupying the place of the left canal of Cuvier, viz./*. Fibrous 
bands beneath the left pleura outside the pericardium, v. Vestigial fold of the peri- 
cardium, 1. Lines or streaks on the back of the left auricles, o. Oblique auricular 
vein, and s. Coronary sinus, g. Great cardiac or coronary vein. h. Vena cava supe- 
rior. 1. Right, and 2, left auricle. 3. Ventricles. 4. Part of left lung. 8. Aorta. 
9. Pulmonary artery. 10. Left pulmonary veins. 11. Cut edge of the pericardium. 
Figs. 1, 2. Heart of an embryo, |^ths of an inch long. Figs. 3, 4. Heart of an em- 
bryo, 1 inch long. Figs. 5, 6. Heart of an embryo, l^ths of an inch long. In these 
figures, viz. 1, 3 and 5, the right canal of Cuvier (c), is seen progressively to become 
shorter, wider and more vertical : the left canal (c'), which is shown opened in figs. 2, 
4 and 6, becomes longer, narrower, and more circuitous in its course to the right 
auricle. 
Fig. 7j left side, and fig. 8, the under surface of the heart of an embryo, 2| inches 
long. The cavities of the heart are filled with hardened blood. The upper occluded 
part of the left canal of Cuvier (c") is well seen ; and also the transformation of its 
