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TEXT-BOOK OF EMBRYOLOGY. 



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 conditions. 

 Such structures are known as tertiary lymph glands. 



The origin of the lymph (plasma) itself is probably extremely complex. At 

 one time it was considered as the result of filtration from the blood plasma 



Afferent lymph, vessels 



Lymph follicle 



Marginal 

 plexus 



Intermediary, 

 plexus 



Trabecula 



Capsule rtf ^~~ 



Efferent lymph, vessels 



FIG. 258. Diagram illustrating a late stage in the development of a lymph gland. 

 Compare with Fig. 257. Stohr. 



through the capillary walls. If lymph originates in this way the filtration is 

 selective, for the chemical composition of the lymph differs from that of the 

 blood plasma. Further researches have tended to indicate that the lymph 

 plasma may be the product of the vital activities of cells. 



The Spleen. 



Since the spleen is generally considered as a lymphatic organ and since re- 

 cent researches have shown that its structure is quite comparable to that of the 

 lymph glands, it seems advisable to consider it under the head of lymphatic 

 organs. Its ultimate origin is not yet settled and the details of its later develop- 

 ment are still obscure. The same difficulties are met with as in the case of 

 the origin and development of blood cells, for it is known that the spleen plays 

 a part in the formation of the blood cells. Its structure differs from that of the 

 lymph glands chiefly in that it possesses no distinct lymphatic sinuses; but it does 

 possess lymph follicles (splenic corpuscles) and densely cellular cords (pulp cords) 

 which are separated by cavernous blood vessels (cavernous veins) . 



