214 RALPH DOUGALL LILLIE 



THE PRIMITIVE BLOOD-CELLS 



The ventral cell mass breaks up into large round cells at about 

 3- to 3.5-mm. body length. The peripheral layer of the splanchnic 

 mesoderm forms an endothelial wall for the ventral sinus. The 

 wall next the yolk mass is incomplete, there being but few 

 endothelium cells on that side, so that the cavity is in part closed 

 in above by the yolk mass itself. This sinus seems to corre- 

 spond to an omphalomesenteric vein, for in later stages it can be 

 traced forward on each side into the sinus venosus. 



Endothelium cells are seen in various parts of the body. These 

 are elongated cells with clear elliptical nuclei containing one or two 

 large nucleoli and but little chromatin. The karyoplasm is clear 

 and colorless. The conspicuous nucleolus is round or oval and 

 stains violet blue with eosin-azure II. The cytoplasm is basophil. 



In these earlier stages the endothelia of the blood-vessels do 

 not seem complete, but present gaps here and there. Mietens 

 ('10) finds this condition also in B. vulgaris. 



In larvae of 3- to 3.5-mm. body length the ventral cell mass 

 becomes resolved into its constituent elements. The cells grad- 

 ually break apart, the mass as a whole enlarges, free space appears 

 within the endothelial wall, and the cells round up into spherical 

 elements which float free in the plasma. Not until about the 

 4 mm. stage are these free rounded cells found in the cavity of 

 the heart or in the systemic vessels. So it would appear that the 

 breaking up of the blood anlage occurs before the commencement 

 of the circulation rather than after. 



The primitive blood-cells (fig. 1, pbc) are large, spherical cells 

 about 13.5 to 22/x in diameter, averaging 17 to 18ju, heavily laden 

 with yolk granules, which are evidently solid in nature, as they 

 are not distorted by mutual pressure, while the nucleus is fre- 

 quently compressed and angular from the resistance of the yolk 

 granules. There are many extra nuclear pigment granules in these 

 and in the endothelium cells. The dark nucleus contains a con- 

 siderable amount of chromatin in angular or rounded masses of 

 varying size on a linin network. The karyoplasm is stained blue 

 with a reddish tinge (eosin-azure II). As a rule, it is darker and 



