EQUIVALENCE OF HEMATOPOIETIC ANLAGES, I. SPLEEN 285 



conception of the blood origin. It cannot, however, be con- 

 sidered as definite proof, because various differentiation products 

 could under different conditions derive from isomorphic but 

 heteropotential cells. The study of the spleen development 

 under influence of stimulation of its stem cells will give further 

 information about causal relation between structural environ- 

 mental conditions and development of the polyvalent stemcells. 



B. HISTOGENESIS IN A CHICK SPLEEN AFTER STIMULATION 



An intense stimulation of the mesenchymal spleen anlage is 

 obtained by means of grafting a tissue mesh of an adult spleen 

 on the allantois of a chick embryo. The photographs on figures 

 1 and 2 demonstrate the enlargement of the host spleen. Both 

 photographs belong to an embryo of 18 days. The spleen is 

 enlarged in both embryos. Wliile the spleen on figure 1 as 

 compared with a normal is enlarged approximately 4 times the 

 spleen on figure 2 shows a much more intense enlargement. The 

 long diameter of the spleen at this stage is about 1-2 mm. After 

 the stimulation it may exceed 1 cm. It is remarkable that the 

 enlargement of the spleen is usually in a visible relation to the 

 intensity of the graft growth. The spleen a corresponds to the 

 graft on figure 3, the spleen b to the graft on figure 4. 



The macroscopical appearance of the enlarged spleen does not 

 give any indication whether the spleen enlargement is due to a 

 local proliferation of the embryonic tissue. This proliferation 

 could be incited by heterogeneous products of metabolism, which 

 from the transplanted and growing cells could penetrate in the 

 vessels and be transported by the blood current into the embryo 

 body. On the other hand, cells from the graft could be trans- 

 ported in the spleen and here under favorable conditions pro- 

 liferate. Tumor like accumulations of tissue would develop 

 in the last instance and enlarge the spleen. The surface of the 

 spleen after stimulation is often covered by protuberancies, 

 numerous small whitish points may be seen on its section. These 

 facts at first suggested rather the idea that the enlargement of the 

 spleen was caused by tumor-like metastases brought by the blood 



