320 GEO. A. THIEL AND HAL DOWNEY 



These vessels can usually not be recognized as arteries until the 

 condensation of the mesenchyme surrounding them has begun. 

 There is no special activity on the part of the endothelium of the 

 arteries, and it is very evident that the syncytium of condensed 

 tissue surrounding them has been derived from the general mes- 

 enchyme of the rudiment. Since there are no ingrowing vessels, 

 there is no tissue to be brought into the organ after it has been 

 established. An occasional lymphocyte proliferating in the tissue 

 after migration from the vessels (Gulland) is of no importance, 

 since the evidence for the derivation of the great bulk of free 

 cells from the fixed mesenchyme is complete. 



Since the free cells in all parts of the organ can be traced back 

 to the mesenchyme of the rudiment, it follows that environmental 

 factors, as claimed by Danchakoff in a series of recent papers, 

 must be of chief importance in determining the line of differen- 

 tiation which the cells will follow. That the 'environment' in the 

 neighborhood of the arteries is different from that of the pulp is 

 evident. 



The writers find that the specialization of cell types begins 

 earlier than was noted by Danchakoff in the chick. In the pig 

 most of the cells liberated from the condensed mesenchyme about 

 the arteries at the tune the lymphoid sheaths are being estab- 

 lished are small, and many of them have dark nuclei with abun- 

 dant chromatin, although the amount and arrangement of this 

 chromatin does not usually correspond exactly to that of the 

 typical small lymphocyte which seems to require a certain period 

 of 'ageing' (Pappenheim) before acquiring all of its character- 

 istic features. Small dark nuclei also appear in many of the 

 mesenchyme cells of this region, which seems to indicate that the 

 differentiation along small lymphocyte lines may begin before the 

 cell is entirely free. There is a noticeable difference in this re- 

 spect in the pulp of these same sections. Here all of the lym- 

 phoid cells are of the large 'hemocytoblast' type, while all of the 

 free small cells belong to the erythrocyte series. The first small 

 lymphocytes are, therefore, derived from the mesenchyme sur- 

 rounding the arteries. 



