144 
MR. MARSHALL ON THE DEVELOPMENT 
The metamorphosis of the pervious vessels on the left side is much more com- 
plicated. 
Outside the 'pericardium. — In one embryo already mentioned (Plate III, fig, 4), the 
left primitive jugular vein was converted into a cord (c"), quite up to the cross branch 
or left innominate vein ; and in that instance no trace whatever could be detected of 
the left cardinal vein. In such cases as the one referred to, the left superior inter- 
costal vein may be subsequently formed through the development of small collateral 
veins, or it may be entirely wanting, — its intercostal branches ending then in some 
other vessels. Usually, however, a piece of the left primitive jugular vein, immediately 
below the cross branch, remains pervious and constitutes the trunk of the left 
superior intercostal vein (fig. 5, /'), which passes over the side of the aorta at a higher- 
point, as the thoi-ax becomes elongated during development. The left cardinal vein 
may be entirely withei-ed, but in some cases (fig. 7) the left primitive jugular remains 
open from the cross branch quite down to the commencement of the canal of Cuvjer, 
where it i-eceives the diminished left cardinal vein (6'), thus metamoi-phosed into the 
lowest bi-anch of an unusually large left superior intercostal vein {i). This vein, be- 
sides intercostal bi-anches, may be very early found to receive thymic, pericardiac and 
mediastinal branches, and often a bronchial vein. In all cases, however, the left car- 
dinal vein is effectually cut off fi-om the occluded left canal of Cuvier (c"), and is lost 
or interrupted at its lower part, so that the left intercostal system becomes broken 
up into several streams. 
JVithin the pericardium. — The pouch-like pervious portion of the left primitive vein, 
seen on the back of the left auricle (Plate III. figs. 4, 5, 6, d ; Plate IV. figs. 7, 8, s), 
is metamorphosed partly into a small oblique auricular vein (Plate IV. figs, 9, 10, 1 1, o), 
and partly into the coronary sinus {s). 
On examining the back of the heart in a series of early embryos (Plate IV.)*, it will 
be seen, as already noticed in the Sheep, that in accordance with a certain twisting 
vdiich takes place in this organ, important peculiarities in length and direction are 
early impressed upon the two canals of Cuvier, which are originally quite symme- 
trical. The right canal (figs. I, 3, 5,c),or future superior cava, passes down perpendi- 
cularly from the neck, becomes widened and shortened, soon reaches the future right 
auricle, and enters by a wide orifice into the upper part of that cavit}^ nearly in a line 
with the inferior cava (e). The left canal (c'), whilst diminishing in size, pursues a 
more circuitous course along the left auricular cavity, beneath the base of the heart, 
to reach the lower and left part of the future right auricle, into which it opens by a 
coiivStricted orifice surrounded by an opake well-defined border. 
Even at a very early period, when this left canal of Cuvier has scarcely begun to 
shrink, its lower part is found, on cutting it open (figs. 2, 4), to be a distinct vascular 
tube, applied to and connected with the back of the future left auricle, but not opening 
into that cavity ; whilst its orifice maintains its connection with the right portion of 
* All the figures in this Plate, excepting fig. 11, are enlarged to two diameters. 
