THORACIC DUCT IN THE CHICK 139 



The application of the term 'blood islands' to the accumulations 

 of cells has been avoided because there is no real condensation 

 of the mesenchyme whereby the differentiating elements fuse to 

 form a solid protoplasmic mass. Each stellate element differen- 

 tiates individually (figs. 1 and 2) and condensation occurs only 

 as a subsequent aggregation of the differentiated cells. The result- 

 ing large mononuclear cells, however, so closely correspond to the 

 first lymphocytes that develop in the area vasculosa of the blasto- 

 derm, the subsequent histories of the cells arising in the two 

 localities being identical, the conclusion is warranted that the 

 differentiated mesenchymal elements in question serve as an 

 intraembryonic source of blood cells. 



While the present investigation has not been of such a nature 

 as to determine whether the mononuclear basophilic cells which 

 have been described give rise to granular leucocytes, they certainly 

 give rise to red blood cells.. The cytoplasm of some of the cells 

 gradually loses its basophilic character and acquires a stronger 

 affinity for the plasma dyes, at the same time becoming quite 

 homogeneous. These changes are concomitant with the addition 

 of hemoglobin. The nuclear changes comprise an increase in the 

 amount of chromatin, its arrangement into a rather heavy retic- 

 ulum and the disappearance of the nucleoli (fig. 2). 



Cells thus modified and containing a moderate amount of 

 hemoglobin, as indicated by the reaction to plasma stains, are 

 erythrobalsts. Without further visible structural changes these 

 acquire more hemoglobin until eventually they are indistinguish- 

 able from the red blood cells in the general circulation. They have 

 therefore become erythrocytes. It is probable also that they are 

 definitive rather than the larger primitive erythrocytes described 

 by Dantschakoff. The changes described above occur both in 

 the cells composing the groups and in the isolated cells, sd that 

 fully developed red blood cells as well as the earlier de\'elop- 

 mental stages are seen not only in the groups but also in the mesen- 

 chymal spaces more or less remote from the groups. 



Among the developing erythi'oblasts, especially in the later 

 stages, are manj^ small cells which apparently go through the same 

 processes of differentiation as the red blood cells themselves and 



