176 



THE ORGANS 



Development. — The first indications of lymph node formation are found in 

 the axilla and groin (pig embryo of about 30 mm.; human toward end of third 

 month) in the connective tissue in which the lymphatic vessels are best developed 

 (p. 170). Here groups of more closel}^ packed cells appear. As the tissue is richly 

 vascular it seems impossible to determine whether the closely packed cells 

 originate within the blood-vessels or develop from fixed connective tissue cells. 

 Each group of cells is the anlage of a lymph node (Fig. loi). The immediately 

 surrounding connective tissue forms the capsule, while the lymph channels just 

 beneath form the subcapsular or marginal sinus (Fig. 102). The point of main 

 connection with outside l)lood-vessels becomes the hilum. As the lymph node 



Afferent lymphatic vessels 



Marginal sinus 



Capsule 



Dense lymph 

 tissue 



Marginal sinus (plexus) 

 Capsule 



Trabecula 



Reticular tissue 



Intermediary 

 plexus 



Efferent lymph, vessel 



Blood vessels 



Fig. 



102. — Diagram Illustrating a Stage (Later Than Fig. 97J in the Development 

 of a Lymph Gland. (Stohr.) 



grows outward, parts of the capsule remain within to form trabeculse while the 

 lymph channels within the nodule apparently develop as ingrowths from the 

 marginal sinus (Figs. loi and 102). 



TECHNIC 



(i) Remove several lymph nodes from one of the lower animals (ox, cat, dog, 

 rabbit), fix in formalin-]\Iiiller"s fluid (technic 6, p. 7), and harden in alcohol. 

 Cut thin sections through the hilum, stain with hsematoxylin-eosin (technic 1, p. 

 20), or with hsematoxylin-picro-acid-fuchsin (technic 3, p. 21), and mount in 

 balsam. 



(2) Expose a chain of lymph nodules {e.g., the cervical or inguinal of a re- 

 cently killed dog or cat). Insert a small cannula or needle into the uppermost 

 node and inject formalin-MUller's fluid until the node becomes tense. By now 

 slightly increasirtg the pressure the fluid may be made to pass into the second 

 node, and so through the entire chain. The nodes are then carefully dissected out 



