EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 287 



a. Changes after stimulation in early stages 



. Transformation of the mesenchymal spleen anlage into a uni- 

 form granulohastic tissue. The study of the normal spleen 

 development has shown that the spleen anlage at the stage of 

 6 to 8 days consists of a more or less dense mesenchymal tissue 

 in the form of syncytium. The oblong nuclei, characterized 

 by the presence of well pronounced nucleoli and scant chro- 

 matic particles lie closely together. At the periphery of the 

 organ the nuclei are more scattered and the tissue looser. 

 The limits of the cells are here also undefinable, for the cells 

 all are united together by numerous short processes. In the 

 peripheral layers of the organ sinuses are already developed, 

 their connection with the veinous circulation is effected and the 

 stimulating substances find easy access into the organ. 



The first effect of the stimulation is exhibited by an intense 

 proliferation of the mesenchymal cells and numerous mitoses. 

 The sinuses spread swiftly over the whole organ and form a net 

 with large meshes. The protoplasm of the mesenchymal 

 syncytium loses soon its uniform structure and becomes less 

 dense; numerous vacuoles appear. At the same time many of 

 the cells become isolated and lose their connection with the 

 plasmodial cell mass. These cells appear intensely basophylic 

 and are very ameboid in their character.^ 



An intense development of lymphoid hemocytoblasts is seen 

 on figure 6. The differentiation of lymphoid hemocytoblasts 

 can assume such proportions, that the greatest part of the mes- 

 enchymal anlage may be converted into free ameboid cells and 

 merely a few mesenchymal cells may remain between them 

 (fig. 20). No regular development of sinuses can take place in 

 such cases. The lymphoid hemocytoblasts multiply intensely, 

 many of them show a beginning differentiation into granu- 

 locytoblasts and acidophylic small granules appear in their 



^ The changes which occur immediately after stimulation at early stages of 

 embryo development merely accelerate and intensify the normal histogenetic 

 processes in the spleen. Therefore illustrations given on plate 3, figures 5 and 6, 

 refer at the same time to normal development as well as to development after 

 stimulation. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 20, NO. 3 



