294 VERA DANCHAKOFF 



b. Changes obtained by stimulation in the stage with defiriite spleen 

 vascularization 



Studying the myeloid metaplasis. Hertz (20) found that besides 

 an intense development of granuloblastic tissue in the pulpa, 

 well pronounced changes in the follicles appeared also. The 

 tissue of the whole follicle might have undergone a differentiation 

 into large lymphocytes (lymphoid hemocy toblasts) . The changes 

 observed in the spleen after stimulation in later stages (at 14 

 to 15 days of incubation) in which the structural peculiarities 

 of the organ are developed, correspond closely to those described 

 by Hertz. The large lymphocytes (lymphoid hemocytoblasts) 

 develop, according to Hertz, in the follicles partly at the expense 

 of the reticulum cells, partly at the expense of small lymphocytes. 

 The development of lymphoid hemocytoblasts at the expense of 

 the mesenchymal reticulum is most evident in the embryonic 

 spleen after stimulation. However, the present experiments offer 

 no evidence that they develop at the expense of small lymph- 

 ocytes, because they appear at a time when the small lymph- 

 ocytes are either still very scant or have not yet developed at all. 



Figure 24 represents the tissue of a follicle 3 days after stimu- 

 lation, applied to a 12 days embryo. The largest part of the 

 cells consists of lymphoid hemocytoblasts, which multiply in- 

 tensely in a hetero- and homoplastic way. The artery walls 

 themselves are usually infiltrated by large basophylic cells, 

 though normally they are formed at this time by a loose tissue, 

 in which free cells are absent. The splitting off of lymphoid 

 hemocytoblasts by the mesenchymal reticulum in such follicles 

 is easily discernible. Though this splitting leads to the 

 development of numerous hemocytoblasts, it however never 

 attains the intensity observed in the pulpa-hke spleen (fig. 20) 

 after stimulation at early stages. Between the ameboid cells 

 a distinct net of mesenchymal cells is apparent and they pro- 

 liferate and continue to split off hemocytoblasts. Endothelial 

 cells of the arteries and their smaller branches seem to be 

 exempt from the stimulating action. Whether the lack of a re- 

 action in the endothelial cells is caused by the final specialization 

 of these cells, or whether the endothelial cells are merely 



