LYMPHATIC SYSTEM 



329 



point of origin by a large sinus, the jugular Ijrmph sac (fig. 352, C). 

 Later the lymph sac reestablishes communication at one or two points 

 in the subclavian-jugular region with the vein. 



The conditions at the posterior part of the body are less certainly 

 known (fig. 352, Z)). A cistern of chyle (a mesenterial lymph sac) 

 and a posterior l3rmph sac develop in close connexion with the post- 

 cava in the region of the nephridial organs, and it is possible that a 

 portion of the thoracic duct grows forward from the cistern of chyle, 

 while other vessels grow into other regions. Later the primitive 



Fig. 353. — Superficial lymph-vesscU of a tadpole of a frog, after Hoycr. 



trunks thus outlined give off branches which gradually extend into 

 all parts of the body, but of their development little is known. 

 Anastomoses occur between the vessels of the two sides of the body 

 and not infrequently the thoracic duct of one side shows more or less 

 degeneration, resulting in a lack of symmetry in the adult. 



Not enough is known of the distribution and development of the 

 chief lymphatic vessels in other animals to render broad generaliza- 

 tions possible, but it can be said that in all groups the system is most 

 extensively developed in the subcutaneous tissue (fig. 353), in the 

 meninges of the central nervous system, in the intermuscular con- 



