168 JOHN STEPHENS LATTA 



the most part, small round cells, with a round nucleus, fairly- 

 dense with chromatin, surrounded by a thin rim of densely 

 basophilic cj^toplasm. These are considered to be true small 

 lymphocytes or late developing stages of the same, for they 

 are morphologically identical, except that the chromatin parti- 

 cles are not always so heavy in the free mesenchymal cells as in 

 the typical adult small lymphocyte. Many developmental stages 

 are found, showing every possible transition between the fixed 

 mesenchymal cell to the free small lymphocyte (figs, 1 and 2). 



The small lymphocytes vary somewhat in size, due, doubtless, 

 to varying sizes of the mesenchymal cells from which they de- 

 velop. Developmental stages between fixed mesenchymal cells 

 and lymphoid hemoblasts are only occasionally seen. It is 

 exceedingly difficult to classify some of the free cells, as many 

 intermediate stages between typical small lymphocytes and 

 lymphoid hemoblasts are found. Typical adult lymphoid hemo- 

 blasts are very rarelj^ found in the lymphatic tissue during the 

 first week of postfetal life. 



The small lymphocytes increase in number quite rapidly, both 

 by transformation of mesenchymal cells and by proliferation of 

 those already formed. JMitotic figures in small lymphocytes 

 are numerous. During the first few^ days of postfetal life, a few 

 mitotic figures can be seen in the ordinary mesenchymal cells also. 



The larger lymphoid hemoblasts do not begin to be present 

 in any quantity until some time during the second week of post- 

 fetal life. When they become present to any extent, frequent 

 mitoses may be seen to occur in them. It seems reasonable to 

 assume that, if these divisions are repeated at frequent enough 

 intervals, the size of the resulting cells would be much decreased; 

 i.e., to the size of a srriall lymphocyte. 



The small lymphocytes, rather than the lymphoid hemoblast, 

 seem here to be the first type to develop. Their origin seems 

 threefold; those first appearing developing only by direct trans- 

 formation from mesenchymal cells and by proliferation of those 

 already formed, and later on appearing also as a result of repeated 

 rapid divisions of lymphoid hemoblasts. 



