468 J. A, BADERTSCHER 



found in the interlobular septa of the thymus in the above named 

 animals. A few were found in the parenchyma of the thymus 

 in a rat embryo 19 mm. in length. The different types of cells 

 named above are derived from lymphocytes and the granules 

 in all of them are products of the cell in which they are contained. 



In the description of the later developmental stages of the 

 thymus mention was made of free nucleated and non-nucleated 

 red blood-cells and eosinophile cells in both the parenchyma and 

 interlobular septa of the thymus. Through the investigations 

 of Maximow ('09 a), and others, who have traced the develop- 

 ment of the blood from early developmental stages in which the 

 cells of the blood islands were still undifferentiated to later stages 

 in which all the different types of blood-cells were found in the 

 circulating blood, the view of a common ancestor for all the 

 different types of blood-cells has been quite generally accepted. 

 This primitive or undifferentiated blood-cell is structurally very 

 much like a large lymphocyte and by some regarded identical with 

 it. Also, Maximow and others have shown that erythrocytes, 

 and granular cells develop from mesenchymal cells of the intra- 

 embryonic mesenchyme.^ 



Since erythrocytes were already present in the blood streams 

 of the youngest embryos collected for this work, the blood islands 

 and other hematopoietic regions were not investigated. In the 

 mesenchyme, however, the development of the free erythrocytes 

 and granular cells was traced apparently from their source. 

 A consideration, therefore, of the source of the above named 

 cells found in the interlobular septa (mesenchyme) of the thymus 

 will be made first, for a knowledge of their origin will aid in 

 determining the origin of free erythrocytes and granular cells 



* For a detailed account of the development of free blood-cells in the mesen- 

 chyme, reference should be made to Maximow's work of 1906. With the methods 

 of technic used for this work I was able to confirm most of his conclusions re- 

 garding the origin of mesenchymal blood-cells. Hence, I have adopted ten- 

 tatively the nomenclature employed by him. A detailed account of my observa- 

 tions would unnecessarily lengthen this article. The descriptions and drawings, 

 therefore, will be only sufficiently detailed to be within the limits of clearness 

 and accuracJ^ The primitive blood-cells or 'WanderzcUen' have been termed 

 'large lymphocytes' throughout this work. 



