298 GEO. S. HUNTINGTON 



ated caudal to the cords of the brachial plexus, enlarges rapidly 

 and becomes distended with clear lymphatic fluid. In embryos 

 between 14 mm. and 16 mm. the resulting axillary or subclavian 

 lymphatic (i4) bag attains relatively enormous proportions, often 

 quite equaling the jugular sac in size. The intercellular mesen- 

 chymal spaces of this entire region likewise appear distended 

 with clear fluid. The cells limiting these spaces become flattened 

 and the entire mesenchyme presents the appearance of a multi- 

 locular spongy reticulmn distended with fluid (fig. 16). Many 

 of these mesenchymal spaces becolne connected with the axillary 

 lymph bag and many others are actually incorporated in it, being 

 largely responsible for its rapid and extreme increase in size. 

 The axillary sac, having lost its early dorsal drainage through 

 the primitive ulnar lymphatic into the jugular sac, now appears 

 as a closed reservoir receiving the interstitial fluids of the body 

 wall and anterior limb bud and storing the same temporarily. 



In the meanwhile the primitive ulnar vein has likewise atro- 

 phied in its proximal cephalic segment (figs. 15 and 16, 5). The 

 venous return from the trunk and anterior extremity is now 

 carried by the newly established channel of the subclavian vein 

 (12), situated caudal and ventral to the brachial plexus (9). 



In this and the succeeding stages numerous isolated and inde- 

 pendent perivenous mesenchymal spaces form along and around 

 the subclavian vein {13). These, the anlages of the future sub- 

 clavian systemic lymphatics, then unite with each other into a 

 rich perivenous lymphatic plexus which distally establishes con- 

 nections with the axillary lymphatic reservoir. In a proximal 

 direction they extend cephalad along the subclavian vein, until 

 they finally unite in a plexiform connection with the ventral 

 prolongation which extends from the subclavian approach of the 

 jugular sac caudad over the ventral face of the jugulo-subclavian 

 venous angle {17), and which also receives the lymphatic chain 

 developed along the course of the internal mammary and thymic 

 veins (fig. 17). As soon as the connections of the axillary sac 

 with the jugular sac have been established through the sub- 

 clavian lymphatics, the reservoir rapidly diminishes in size. This 

 stage is usually reached in embryos of 21 mm. to 22 mm., but 



