THE LYMPHATIC SYSTEM. 105 



blood-spaces of the liver directly to the heart. This makes a very direct channel, a 

 more direct one than existed previously when the blood from the subcardinal came to 

 join that of the cardinal, passing up to the common cardinal and then back to the 

 heart. The new channel through the liver rapidly enlarges and becomes recogniz- 

 able as the vena cava inferior. This important venous trunk is a combined vessel, 

 comprising, first, a part of the hepatic vein; second, a large channel developed 

 from the sinusoids of the liver; third, the upper part of the right subcardinal vein, 

 and, fourth, the lower part of the right cardinal. 



The Lymphatic System. 



The lymph vessels arise in connection with the veins and are probably out- 

 growths of the vascular endothelium, although some authorities state that they may 

 begin as sub-endothelial spaces. They may be recognized by their very delicate 

 but distinct endothelial walls, thus differing from the mesenchymal spaces-, and by 

 the complete or almost complete absence of blood-corpuscles within them. They 

 probably end blindly. They appear relatively late, for they do not develop until 

 after the limb buds are well advanced (pigs of 14 mm.). The first lymph-vessels 

 develop from the anterior cardinals in the cervical region, and rapidly fuse to 

 make a pair of very large vesicles, the jugular lymph-sacs (Fig. 60, S.l.j], which 

 are closely applied to the veins. Each jugular lymph-sac empties into the cardinal 

 vein near its junction with the subclavian through a valve-like orifice. Whether 

 this connection is primary or secondary is still uncertain. From each sac, narrow 

 vessels bud out into the mesenchyma, anastomose with one another, and, by spread- 

 ing more and more, produce the lymphatic system of the neck and head. Sub- 

 sequently, the jugular sac is resolved into the deep cervical plexus' of lymphatics. 

 Similar lymph-sprouts in slightly older embryos produce a. more irregular median 

 mesenteric sac (Fig. 60, S.l.m) just below the renal anastomosis, R.A; and also 

 another sac, Cis, dorsal to the first. The mesenteric sac sends branches into the 

 mesentery to drain the intestine, and at the same time joins the dorsal sac which 

 enlarges, becoming the cisterna chyli. Still later there are formed in a similar 

 manner two smaller sacs of dense plexuses, termed the sciatic, for they develop in 

 connection with the root of the sciatic vein (Fig. 60, Sci). The sciatic sacs are 

 produced later than the other four. Like the jugular, the four later formed sacs 

 serve as centers of growth for the lymph-vessels. In addition, other lymphatics 

 develop from other veins. Especially notable are the sprouts from the azygos 

 vein, which unite and ultimately give origin to the main lymphatic trunk, the 

 ductus thoracicus, which joins tailward the cisterna, headward the left jugular sac. 



The lymph-glands make their first appearance considerably later than the 

 vessels (rabbits, 25-30 mm.; human embryos, 30-45 mm.). Each appears be- 

 tween a vein and a lymph-vessel and is recognizable as mesenchyma crowded 

 with young leucocytes. The glands are very small at first, but are quite sharply 

 circumscribed. 



