194 BLOOD AND BLOOD-FORMING ORGANS. 



cytes). It piay be readily understood that the latter function of the 

 leucocytes is of the greatest importance in certain pathologic pro- 

 cesses. 



It is somewhat venturesome at the present state of our knowl- 

 edge to make definite statements as to the origin in postembryonic 

 life of the various forms of white blood-cells "above described. The 

 following statement, however, seems warranted from the evidence 

 at hand. 



The lymphocytes would seem to be developed in the meshes of 

 adenoid tissue, especially in the so-called germ centers of Flemming, 

 m the adenoid tissue of lymph-glands and lymph-follicles (see under 

 these). Here the cells undergo active karyokinetic division, but 

 where the cells which pass through the process originate is a matter 

 concerning which there is a difference of Opinion. Some investi- 

 gators believe that they penetrate the germ centers with the lymph, 

 and find there a suitable place for division. Again, others see in 

 Flemming's germ centers permanent organs whose elements remain 

 stationary and supply the blood with a continuous quota of lympho- 

 cytes. Be this as it may, the fact remains that the germ centers 

 are the most important regions for the formation of lymphocytes. 

 From these they pass out with the lymph current into the blood 

 circulation, or directly into the blood-vessels, there to enter upon the 

 functions which they are called upon to perform. The leucocytes 

 with neutrophile granules are probably developed in the blood and 

 lymph from mononuclear leucocytes which have their origin in the 

 spleen pulp, possibly also in the bone-marrow. The leucocytes of 

 circulating blood with eosinophile granules in all probability come 

 from mononuclear cells with such granules found in bone-marrow. 

 Under certain conditions it would seem that they also develop in 

 connective tissue. The leucocytes with the basophile granules prob- 

 ably enter the circulation from the connective tissue of certain re- 

 gions. The lymphocytes and leucocytes found in the blood are also 

 found in the lymph-vessels and lymph-spaces. 



4. BLOOD PLATELETS-THROMBOCYTES. 



The third element of the blood is the blood platelets (Bizzozero) 

 (blood-placques, Laker ; hematoblasts , Hayem ; thrombocytes, Deck- 

 huysen). They are extremely delicate and transitory structures, whose 

 existence in the living blood was denied for a long time by many in- 

 vestigators, but whose presence in the wing vessels of the living bat 

 was conclusively demonstrated by Laker (84). They are free from 

 hemoglobin, are of round or oval shape, and in mammals measure 

 about 3 fJL in diameter. Owing to the fact that they readily clump 

 together when blood leaves the vessels, and undergo change, it is 

 somewhat difficult to give an estimate of their number. They are 

 said to be present in human blood to the extent of 200,000 to 

 300,000 in every cubic millimeter. By the exercise of great care 



