THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



281 



ipheral 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 trabecules. The blood vessels tend to lie in the 

 trabecular, but a small branch probably passes to each follicle. In the follicles 

 themselves the lymphocytes proliferate and the central part of each follicle 

 becomes a germinal center. The connective tissue among the lymphatic vessels 

 composing the marginal plexus becomes proportionately less as the vessels 

 enlarge and finally exists only as strands of reticular tissue which, naturally, are 

 covered by the endothelium; thus the marginal plexus becomes the marginal 

 sinus. The intermediary sinus is formed by the channels which originally in- 

 vaded the cell mass. The reticular tissue is probably composed of remnants 



Afferent lymphatic vessels 



Marginal sinus 



Capsule 



Dense lymph 

 tissue 



■Marginal sinus (plexus) 



Capsule 

 Trabecula 

 Reticular 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. Sldhr. 



of the original connective tissue. All the channels converge at the hilus to 

 form the efferent lymphatic vessels (Figs. 257 and 258). 



The hamiolymph glands are probably developed in much the same manner 

 as the lymph glands except that in the former the sinuses are composed of blood 

 vessels instead of lymphatic vessels. 



The first lymph glands to develop are those in the axilla, in the inguinal 

 region, in the neck, and in the base of the mesentery. These are the so-called 

 primary glands and develop during fcetal life. They are of constant occur- 

 rence in these regions, but vary in number in different individuals. The 

 secondary lymph glands are those in the bend of the elbow, in the popliteal space, 

 in the mesentery, and around the aorta. Some of these develop during 



