WHITE BLOOD COKPUSCLES 79 



In later fetal life, as in the adult, the formation of white blood 

 cells takes place actively in all the lymphoid organs and tissues. 

 Those cells which are found in the germinal centers of the lym- 

 phatic nodules are especially active ; their cell reproduction is by 

 mitosis, the daughter cell, according to Fleming, being of the 

 lymphocyte type. 



The presence of centrosomes and mitotic figures in all varieties 

 of leucocytes, both in the adenoid tissues and under certain con- 

 ditions in the blood as well, has been so constantly found as to 

 indicate that this, rather than amitosis as was formerly supposed, 

 is perhaps the only method of cell division by which white blood 

 cells are reproduced. The same fact would indicate that each 

 variety of leucocyte when once established is capable of repro- 

 ducing itself. 



As soon as leucocytes appear in the fetal blood current all of 

 the several varieties can be distinguished. It is therefore doubt- 

 ful if one variety can in any way be regarded as a more mature 

 form than the other. 



While in fetal life all varieties of leucocyte may be formed in 

 any lymphoid organ, yet with the appearance of the bone marrow 

 the formation of the granular varieties, eosinophiles, neutrophiles, 

 and basophiles, appears to become most active in this tissue. It 

 is possible that the marrow is the only tissue in which these cells 

 are reproduced in adult life (Ehrlich). The mononuclear non- 

 granular types lymphocytes and large mononocular cells con- 

 tinue to be actively regenerated in the lymphoid organs, i. e., the 

 lymphatic nodes, lymphatic nodules, and spleen. 



The finding of large giant cells, megakaryocytes, resembling 

 the osteoclasts, but differing therefrom in that the former pos- 

 sess a single polylobar nucleus, whereas the osteoclast is multinu- 

 clear, each nucleus being of ellipsoid shape and of approximately 

 equal size in all hemopoietic tissues, has been taken to indicate 



anlage of the thymus primitive leucocytes are formed by mitotic division of cells 

 which are apparently derived from the epithelium of the gill clefts. Working 

 independently, Nussbaum also found that the anlage of the thymus in fishes 

 was derived from the epithelium of the primitive gill clefts. These observa- 

 tions would therefore indicate a possible ectoblastic origin for the leucocytes, 

 and that the earliest cells of this type, as later in life, are derived from the lym- 

 phoid organs, and only by their characteristic nomadic tendency do they gain 

 admission to the primitive blood vessels to be thereby distributed to distant 

 portions of the fetal body. These observations, however, lack further con- 

 firmation. 



