138 
MR. MARSHALL ON THE DEVELOPMENT 
hardened blood (fig. 5, d). Even in embryos as long as ^ths of an inch (fig. 4), no 
red line could be traced quite across the neck, although the lateral primitive veins 
were turgid with blood, and presented two conical projections at the situation of the 
cross branch. 
This cross branch is supposed to be formed by the enlargement of a previously 
developed vessel of almost capillary dimensions, perhaps one of several such vessels 
passing across the neck ; but it is possible that it might be formed in the same way 
as the other great vessels. The appearances above described would suggest the idea 
that this transverse branch was formed by the budding out and subsequent junction 
of two opposite points of the older veins, but this appearance is probably owing 
merely to the vessel being smaller and empty of blood in the middle of its course, or 
to its being accidentally broken at that point ; but still it would seem that the extre- 
mities of the new transverse vein in connection with the older vessels are at first 
always enlarged more than the intermediate portion. In recent embryos of the Guinea 
Pig, measuring about half an inch in length, I have twice recognized the transverse 
branch as a very minute continuous vessel, passing quite across the neck, between the 
two jugular veins, just above the pericardium. In one very early embryo (-^ths of 
an inch long), a small vessel, evidently a vein, traversed the neck higher up, supported 
by the branchial arch which was being transformed into the lower jaw; the lower 
cross branch was not yet formed in this case. 
During the widening of the cross branch, the two jugular trunks {aa!) at that point 
gradually approach each other, — the distance between them, absolutely as well as 
relatively, decreasing (compare figs. 2 and 6, representing embryos of -^thsand^ths 
of an inch in length). Instead of descending parallel with each other, these veins now 
incline to the middle line of the neck, opposite the cross branch, and assisted by the 
shortening and widening out of that vessel, they appear at length to coalesce, almost 
by lateral adaptation, before any diminution of the left primitive vein has begun to 
take place (fig. 7, embryo 1 inch ^ths long). 
In later embryos (fig. 8, 1 inch and -^ths long; fig. 10, 1|- inch long), in which 
the neck is already becoming elongated, by the time that the occlusion of the left 
primitive vein is accomplished, the two jugular veins, having received the large super- 
ficial veins of the neck and those from the anterior limbs, are so closely applied to 
each other, and their connecting branch is become so entirely absorbed into them, 
that they join together at a very acute angle; or rather, the vein of the left side, 
now the left innominate vein, runs continuously into the lower part of the right 
primitive vein, or superior vena cava, whilst the right innominate vein appears to fall 
into this large continuous trunk at an acute angle. 
At still later periods, when the vertebral column projects prominently forward at 
the lower end of the slender neck, immediately above the narrow aperture of the 
thorax, this obliquity of the junction between the innominate veins is as marked as 
in the adult animal. 
