474 J. A. BADERTSCHER 



red cells are found among the erythrocytes, while other groups 

 are composed almost entirely of nucleated red cells. Both the 

 erythrocytes and nucleated red blood-cells vary in size, but the 

 small ones greatly predominate over the medium sized and larger 

 ones. They are usually irregular in outline. The structure 

 of the nuclei vary as those in the 55 mm. stage. An interesting 

 and most helpful feature in tracing out the origin of erythrocytes 

 in late developmental stages is the presence of lymphocytes 

 with granular nuclei, the structure of which is the same as that 

 of the erythroblasts and normoblasts found in the interlobular 

 septa, the only difference being their smaller size. Only a few 

 small lymphocytes with this type of nucleus were found in the 

 thymus of the 55 mm. stage and in the interlobular septa of the 

 125 mm. embryo. They stain intensely and when examined 

 with lenses of low magnification appear as black dots in con- 

 trast to the other small lymphocytes among which they lie. 

 Maximow ('09 b) in writing of the erythropoetic function of 

 the thymus, which he denies, makes mention of this type of 

 lymphocytes but on the ground that they contained no hemo- 

 globin he does not consider them to be normoblasts. It is true 

 that in many lymphocytes with this type of nucleus, some- 

 times entire groups, no traces ol hemoglobin can be detected in 

 their cytoplasm even when highly magnified (X 2000). But, 

 many small cells can also be found with similarly granular 

 nuclei and with a distinct reddish tinge which indicates the 

 presence of hemoglobin in their cytoplasm. These are small 

 erythroblasts that are derived from small lymphocytes and the 

 small cells referred to by Maximow, and so plentifully found 

 in the thymus of this developmental stage, are transition forms 

 between the ordinary small lymphocytes and the small erythro- 

 blasts. On account of the meagre amount of cytoplasm in these 

 erythroblasts they appear much as if the nucleoplasm was 

 stained slightly red. But that is not the case for the red stained 

 cytoplasm, of those in the late normoblast stage in which the 

 nucleus has become shrunken and pyknotic, stands out sharply, 

 although it is small in amount. Transition forms are often 

 scattered along the border of groups of red cells containing many 



