THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



251 



liferate 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 invaded the cell mass. The 

 reticular tissue is probably composed of remnants of the original connective 

 tissue. All the channels converge at the hilus to form the efferent lymphatic 

 vessels (Figs. 219 and 220). 



The haemolymph glands are probably developed in much the same 



Afferent lymph, vessels 



Lymph follicle 



Marginal 

 plexus 



Intermediary, 

 plexus 



Medullary cord 



Trabecula 



Capsule r .^ s ^ 



Efferent lymph, vessels 



FIG. 220. Diagram illustrating a late stage in the development of a lymph gland. 

 Compare with Fig. 219. Stohr. 



manner as the lymph glands except that in the former the sinuses are filled 

 with red blood cells. 



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 

 foetal life and some later. While lymph glands are of constant occurrence in 

 some regions throughout life, the number may vary at different times in any 

 region; and there may also be variations in different individuals. Glands 

 may be called into existence at any time during life, in almost any region, 

 as the result of exceptional activity of some organ, or in pathological con- 

 ditions. Such structures are known as tertiary lymph glands. 



