THE DEVELOPMENT OF THE VASCULAR SYSTEM. 279 



the mesenchyme adjacent to the large venous trunks and communicate with the 

 latter only secondarily. The first traces of the system appear in cat embryos 

 along the anterior cardinals (jugulars) as unconnected spaces; these later unite 

 to form the cervical "lymph hearts," and the latter then acquire connections 

 with the cardinals. Similar spaces appear in the vicinity of other veins, and 

 subsequently all the spaces unite to form the complex system of lymphatic 

 vessels. Some of the vessels enlarge to form the ducts, while others constitute 

 the branches and the general network of smaller lymphatics. An interesting 

 point is that the lymphatics develop more rapidly around the veins which are 



. Cellular mass 



^ Mesenchyme 



ff~--- Marginal plexus 



Anlagen of _. ..[.-. ....y..-..~" 1. --,- - V '-'-. jt 'T- '"*' , 



mph glands ""vV-V. ' -f . * ' 4 'Vk . V . .-, O; 



V 



FIG. 255. From a section through the axilla of a human embryo of 106 mm. 

 (about 4 months), showing anlagen of lymph glands. Kling. 



destined ultimately to atrophy. This affords one explanation of the fact that 

 the thoracic duct is on the left side, for the veins (cardinals and subcardinals) on 

 the left side atrophy to a greater extent than those on the right (see Fig. 236). 



The Lymph Glands. 



The lymph glands do not begin to develop for some time after the lymphatic 

 vessels, since there are no indications of them in the human f cetus until the latter 

 part of the third month and none in pig embryos until they have reached a 

 length of 30 mm. While it is definitely settled that lymph glands originate 

 in very close relation with the lymphatic vessels, certain points in their later 

 development need further study. In the axilla and groin, for example, the 

 lymphatic vessels form a dense network in the meshes of which are masses of 

 connective tissue. These masses become more cellular and with the surround- 



