EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 275 



Tonkoff (41) in 1900. The spleen anlage appears in an embryo 

 nearly 4 days old — in the mesenterium dorsale duodeni in the 

 region of pancreas dorsale. It is distinctly separated from the 

 coelomic epithelium, which surrounds it. In the I stage of its 

 appearance the spleen anlage is purely mesenchymal. It is 

 distinguished from the surrounding mesenchymal tissue by the 

 denser appearance of the tissue. At this time mesenchymal 

 cells of the anlage present short ramifications, which soon are 

 lost, the cells multiplying intensely and joining finally in a common 

 syncytium (fig. 5 and 6). The spleen anlage at early stages is 

 identified rather by its localization than by the character of its 

 cells. Similar agglomerations of syncytium-like mesenchyme are 

 encountered in many other places, and their further development 

 exhibits a great analogy with that of the spleen anlage, resulting 

 in a differentiation into lymphatic tissue. The more intense 

 proHferation of the mesenchyme in the region of the spleen is 

 evidently due to local favorable conditions. The development 

 of the spleen at the expense of mesenchymal cells without any 

 relation to the endoderm nor to the coelomic epithelium may be 

 regarded as a well-founded fact. 



The appearance in the spleen anlage of numerous ameboid 

 cells, the lymphoid hemacytoblasts, and their differentiation 

 into granulocytoblasts and into small lymphocytes was described 

 by Danchakoff (9) Cl6a), in Tropidonotus natrix. The fact 

 that the small lymphocytes develop in the spleen in later stages 

 was also noticed. However, it is improbable that the different 

 stages in themselves should be accounted for as differentiating 

 factors. If the small lymphocytes do appear later, the granulo- 

 cytoblasts nevertheless continue to differentiate in the spleen, at 

 least in embryonic life. Conditions for both lines of differentiation 

 must therefore coexist in later stages. The differentiating 

 factors should be sought rather in different structural conditions, 

 appearing at definite stages, and determining from the time of 

 their appearance the lines of differentiation of the polyvalent 

 stem cells. 



Since the structural peculiarities are exhibited in a more 

 striking manner in the spleen of an adult or a young chick, it 



