DEVELOPMENT OF THE MAMMALIAN SPLEEN 313 



into granulocytes is no longer evident. Thus the arteries and 

 their branches become surrounded by areas of mesenchyme cells 

 which soon appear as islands of mesenchymal tissue which 

 occupy the interstices between the arteries and the earl}^ pulp of 

 the organ. These islands constitute the rudiments of the folli- 

 cles. The lymphoid hemocytoblasts that are in the islands shoAv 

 intense prohferative activity, resulting in daughter cells having 

 the character of 'dwarf hemocytoblasts' which are further differ- 

 entiated into true small lymphocytes. The small lymphocytes 

 are present both in the red and the white pulp and are morpho- 

 logically identical in both regions. 



According to the above description, 'dwarf hemocytoblasts' 

 are the essential precursors of the true small lymphocytes; they 

 are produced by rapid proliferation of lymphoid hemocytoblasts, 

 the daughter cells of which never reach the large lymphocyte stage. 



The early lymphoid arterial sheaths of the pig contain very 

 few large lymphocytes (hemocytoblasts) and mitotic figures are 

 scarce. The great majority of the small lymphocytes are, there- 

 fore, cut off from the mesenchyme without passing through the 

 hemocytoblast stage. Additional evidence for this view is fur- 

 nished by the large number of intermediate forms which lead from 

 the tvpical small Ivmphocyte to the fixed mesenchyme cell (fig. 

 7). ^ 



Van der Stricht sees the small lymphocytes formed during the 

 later part of embryonic life and attributes their increase in num- 

 ber to cell division, and not to a direct transformation of reticular 

 cells. 



Jolly, in a study of the development of the spleen of the white 

 rat, found the small lymphocytes arising from the 'primitive 

 spleen cells' which he describes as large cells with a slightly baso- 

 philic cj^toplasm and large clear nuclei. 'Lymphocytes' were 

 not seen, however, until near the time of birth. In developing 

 lymph nodes he saw the same type of large lymphoid cells formed 

 by transformation of the mesenchyme tissue. Maximow reports 

 the formation of small lymphocytes from fixed cells in developing 

 lymjohatic nodules. These fixed cells are very small cells which 

 round off and become ameboid, when they appear either as 



