The Lymphatic System in Human Embryos. 87 



other two show the cisterna chyli well with large connections with the 

 mesenteric sac. This is especially true in No, 84, when the series 

 is transverse, the sections looking like Fig. 9 of Mr. Baetjer's series. 

 Both the cisterna chyli and the retroperitoneal sac are easily located 

 from Eig. 11. They are bridged from the very beginning. 



These four series, however, are much more interesting in connec- 

 tion with the posterior lymph sacs. As we have seen, these sacs 

 begin in an embryo about 24 mm. long as sacs along the primitive 

 sciatic veins. In an embryo of 30 mm. they are long, narrow sacs. 

 In Fig. 17 it will be seen that in an embryo 50 mm. long they have 

 become large sacs lying in the side of the pelvis opposite the first 

 three sacral vertebrse. The entire dorsal wall of the sac is occupied 

 by a lymph node. In one of the other series it is plain that the sac 

 is opposite the bifurcation of the vein into the sciatic and femoral 

 veins, and that there is a large lymph node in the angle of these two 

 veins. From the sacs extend vessels, both along the femoral vein, as 

 shown in the figure and along the sciatic; both of these groups of 

 vessels have developing lymph nodes. These are secondary nodes 

 in contrast with the primary nodes which come from the sacs.. The 

 primary groups of nodes are the jugular, subclavian, retroperitoneal 

 and posterior. The early secondary nodes are near the sacs, a point 

 also in support of the outgrowth of lymphatics from centre to 

 periphery. 



The last embryo of the series (No. 172), measuring 80 mm., is 

 especially valuable in connection with the fate of the jugular lymph 

 sacs, the development of lymph nodes and the spread of the peripheral 

 lymphatics. The jugular sac is fast becoming transformed into a 

 large group of lymph nodes. In a few sections there are remnants 

 of the sac measuring 1.75 x .5 or even 1.75 x 1 mm., but most of the 

 sac has disappeared. There are also secondary lymph nodes along 

 the other veins of the neck, for example along the external jugular 

 vein next the parotid gland, and along the facial vein at the angle 

 of the jaw. 



In connection with the arm there is an extensive group of nodes 

 over the shoulder. In the axilla there are four groups — one posterior 

 to the vessels and nerves, one along the subclavian vein, and two 

 groups anterior to the pectoralis minor muscle. 



