106 The Development of the Lymphatie System in Rabbits 
obscure spaces which appear to be lymphatic, along the aorta, and in re- 
lation with the azygos veins. An occasional apparent connection with 
the vein suggests their venous origin in situ. The mesenteric and sub- 
cardinal plexuses have united with one another. ‘They do not empty into 
the veins. The subcardinal sacs extend from the renal anastomosis 
almost to the sciatic vein, connecting with one another across the median 
line, as in the pig. No lymphatic vessels follow the ilo-lumbar veins 
into the posterior body wall. 
Returning to the rabbit embryos it will be seen that Fig. 7 from a 
21 mm. rabbit differs from Fig. 4, the 14.5 mm. embryo, chiefly in regard 
to the thoracic duct. The duct is represented by a pair of vessels which 
connect with one another and pass on to the left jugular sac. Sometimes 
in the adult rabbit, as figured by Gage (02, p. 650), and occasionally in 
man, the thoracic duct bifurcates anteriorly and passes to the jugular 
sacs on either side. This did not occur in the 21 mm. embryo, which 
exhibited the relations figured by Sabin, Vol. I, p. 383. 
In Fig. 7 scattered lymphatics are shown along the external jugular 
vein and its branches. One much larger than the rest occurs where the 
anterior and posterior facial veins unite. From its isolation it probably 
arose independently of the large jugular sac. Other and more isolated 
lymphatic centers are seen in the oldest rabbit studied, one of 20 days, 
29 mm., Fig. 8, notably along the pudic and the sciatic veins. ‘They arise 
near the junction of several venous branches, with which, however, they 
are not in communication. 
In the oldest embryo the lymphatic system has invaded the skin to such 
an extent that it is impracticable to represent more than a small part of 
it. In entering the skin the lymph vessels accompany the veins, those 
of the head following chiefly the external jugular vein. The jugular sac 
has become relatively less important, and persists as the lymphatic sheath 
of the internal jugular vein. The deep subcutaneous extension has 
become covered by a thin layer of muscle, presumably the panniculus, 
and does not appear to connect with the more superficial vessels of the 
skin. There are no lymphatics in the distal part of the arm, but the sub- 
cutaneous vessels of the shoulder are attended by rich networks. These 
veins are the external mammary, and another which is ventral to the 
scapula and posterior to the shoulder joint,—a subscapular vein. The 
lymphatics along this large subscapular vein do not connect with the 
jugular sac. At the point Z. N., indicated in the figure, a small but 
very distinct lymph node has developed in relation to these subscapular 
lymphatics. A corresponding node is found on the opposite side of the 
body. 
