218 RALPH DOUG ALL LILLIE 



THE STAGE OF DIFFERENTIATION OF THE PRIMITIVE BLOOD- 

 CELLS INTO PRIMITIVE ERYTHROBLASTS AND LARGE 

 LYMPHOCYTES. THE LOSS OF THE YOLK 



In larvae of 5- to 6-mm. body length the primitive blood-cells 

 lose their yolk almost completely, and differentiate into two new 

 types, one of which is a large, oval, flattened cell with much vacu- 

 olated oxyphil cytoplasm, the primitive erythroblast, the other 

 is morphologically identical with the large lymphocyte (fig. 3, 

 p.eb, Lib, Lie). 



The yolk granules seem to disappear by intracellular solution, 

 not by granular fragmentation. Vacuoles appear around their 

 margins, enlarge and coalesce, while the yolk granule decreases 

 in size, finally passing into the interior of the vacuole. The 

 yolk granules become paler as they diminish and finally disappear 

 completely. The contents of the vacuoles are clear and colorless, 

 not staining with eosin-azure II. Bryce ('04) suggests that the 

 yolk granules, when they disappear, may break up into granules 

 which are eosinophil in nature, and that the early leucocytes 

 may be concerned in the distribution of yolk material. The 

 latter statement may be true, but as to the former, all yolk has 

 long since disappeared from the blood-cells before the first eosin- 

 ophil granulocyte appears in Bufo. We must note, however, that 

 Bryce lacked stages which showed the complete details of the 

 loss of the yolk, which might modify his interpretation. A third 

 alternative exists: that the yolk granules may be extruded from 

 the cells into the circulating blood, since one frequently sees 

 apparently free granules in the blood stream. Usually, however, 

 a small amount of basophil cytoplasm or a few pigment granules 

 may be seen close to them, so it may be that they lie within a 

 pseudopod of a cell in the next section. It seems perfectly pos- 

 sible, however, that the granules may be extruded from the cells 

 and digested by the proteolytic enzymes of the blood. 



Mietens ('10) merely states that the yolk disappears. Bryce 

 ('04) notes that a vacuolization of the cytoplasm occurs and 

 seems to be associated with the disappearance of the yolk not 

 only in the blood-cells, but in the general tissue cells of the body 

 as well. I have noted such an appearance in the mesenchyme 

 cells of Bufo. 



