304. George S. Huntington and Charles F. W. McClure. 
precaval veins (precardinal) anastomose with each other across 
the median line and thereby establish the anlage of the definite 
right and left innominate (brachio-cephalic) veins. In the 18 mm. 
embryo this anastomosus is completed and the blood from the 
left jugular and left subclavian districts is now, for the most part, 
directed into the right precava. As the result of this transfer the 
right precava in the 18 mm. embryo has become much enlarged and 
the left correspondingly reduced. 
In addition to the innominate anastomosis, mentioned above, 
another anastomosis occurs in this region ventral to the thymus 
gland. This makes its appearance in the 15 and 16 mm. embryos, 
and before the definite innominate anastomosis has been estab- 
lished; but with an increase in size of the latter, this anas- 
tomosis soon disappears. In the 18 mm. embryo, however, 
the thymus is still crossed ventrally by a net-work of vessels 
which represents the remains of this sub-thymic anastomosis. 
The transferal of the blood current from the left jugular and 
left subclavian veins to the right precava is illustrated by figs. 13, 
14, 15, 16 and 17 to which the reader is referred. An examination 
of these figures will make plain the conditions of the venous sys- 
tem in this (figs. 64 and 65), and in the embryo next to be con- 
sidered (25 mm., fig. 66). 
Lymphatic development has reached an advanced stage in the 
18mm. embryo. The capacious jugular sac of each side has estab- 
lished in this instance the typical taps at the common jugular and 
jugulo-subclavian angles, respectively. (Cf. fig. 3 of adult). The 
left jugular sac also receives the large thoracic duct on its dorsal 
aspect. The lateral view of the left side of the reconstruction 
(fig. 65), shows this connection. 
The jugular sacs present further evidence of the reduction of 
the ventral portion, the main sac being formed by the dorsal 
part of the earlier lymphatic ring surrounding the first four spinal 
nerves. Asa result of this change, with the exception of the fifth 
nerve (fig. 64,SP.N.V), which still penetrates through the sac, the 
remaining four anterior nerves pass between the sac and the internal 
jugular vein and the lymphatics which accompany that vessel. 
The branch of the thyro-cervical artery which extends dorsad 
