THE DEVELOPMENT OF THE VASCULAR SYSTEM 



281 



network around the mass. This network is the marginal plexus, and it 

 communicates freely with the neighboring lymphatic channels. Within 

 the mass of cells blood vessels are present from the beginning, and these 

 are destined to be the blood vessels of the lymph gland, and the point of their 

 entrance and exit marks the hilus. Outside of the marginal plexus the con- 

 nective tissue condenses to form the capsule. The gland at this stage thus 

 consists of a central compact cellular mass, made up of connective tissue 

 and lymphocytes, in which blood vessels ramify; a plexus of lymphatic 

 channels around the mass which communicate with the neighboring channels ; 

 and around the whole structure a capsule of connective tissue (Fig. 256). 

 Further development consists of the breaking up of the cell mass by 



Afferent lymphatic vessels 



Marginal sinus 



Dense lymph, 

 tissue 



Intermediary 

 plexus 



Efferent lymph, vessel 



Blood vessels 



FIG. 257. Diagram illustrating a stage (later than Fig. 256) in the development 

 of a lymph gland. Stohr. 



lymphatic channels and the formation of the follicles. It seems probable 

 that branches from the marginal plexus invade the cell mass principally 

 from an area around the hilus, thus breaking it up into smaller irregular 

 masses or cords. At the side opposite the hilus the invading channels are less 

 numerous, leaving larger parts of the mass which become the follicles (nodules) 

 of the cortex. On all sides the invading channels communicate with the 

 marginal plexus and form the so-called intermediary plexus. The gland as 

 a whole enlarges and its peripheral part pushes outward into the surrounding 

 tissue. Over the follicles the capsule is pushed outward, while between them 

 it remains in place and comes to dip into the gland as the trabecula. The 

 blood vessels tend to lie in the trabeculae, but a small branch probably 

 passes to each follicle. In the follicles themselves the lymphocytes pro- 



