HISTOGENESIS OF BLOOD IN BUFO HALOPHILUS 215 



the chromatin is more abundant than in the endothelium cells, 

 but transition forms are found which approach the nuclear con- 

 stitution of the latter. The amount of chromatin does not show 

 any definite relation to the degree of basophilia of the cytoplasm. 

 The chromatin, when stained with thionin, appears as angular or 

 rounded blocks of an intensely dark blue color. The large round 

 nucleolus or nucleoli, they being frequently two in number, stains 

 bluish violet with eosin-azure II, and reddish violet, i.e., meta- 

 chromatically, with thionin. With the former stain it shows 

 little color difference from chromatin itself, and, as will be seen, 

 this difference becomes less and less apparent as development 

 proceeds. The cytoplasm is colorless or slightly basophil. The 

 basophilia appears especially around the periphery of the cell. 

 In some cells no basophilia is evident, though observation is 

 difficult on account of the large volume of yolk, but in those cells 

 in which the yolk is decreasing in amount considerable areas of 

 cytoplasm may be observed. When in mitosis these primitive 

 blood-cells show larger expanses of yolk-free cytoplasm, and but 

 little basophilia is then evident. 



In these stages, 3.5- to 4.5-mm. body length, the mesenchyme 

 cells (fig. 2, ms) become almost free from yolk. Occasional 

 rounded cells (fig. 2, pbc), rich in yolk and morphologically iden- 

 tical with the primitive blood-cells, are seen in the loose mesen- 

 chyme. Two possibilities exist regarding them: that they have 

 lagged behind the spindle cells in yolk elimination, their nuclei 

 being modified so as to resemble those of the primitive blood-cells, 

 or that they are primitive blood-cells which have escaped from 

 the vessels and wandered out into the mesenchyme. The second 

 is not excluded, but the first seems more probable, for such cells 

 are found in the head mesenchyme before the breaking up of the 

 ventral cell mass, while the mesenchyme cells are still yolk-laden, 

 but are taking on spindle forms with diminishing chromatin 

 content of their nuclei. 



The steady decrease in food yolk is more evident toward the 

 5-mm. stage, the larger cells becoming elongated and oval, with 

 a more circular cross-section, and with this diminution a centriole 

 sometimes becoming visible in the increasingly basophil cytoplasm. 



THE AMERICAN JOURNAL OP ANATOMY, VOL. 26, NO. 2 



