220 EALPH DOUGALL LILLIE 



scattered particles and in occasional isolated strands. A large 

 smoothly rounded, metachromatically basophil nucleolus persists, 

 showing but little change from the preceding stage. The cell is 

 now a typical large lymphocyte. 



Such cells arise also from other sources than the primitive 

 blood-cells of the blood stream. Endocardial cells bulge out into 

 the heart cavity and become Constricted from the endocardium, 

 at the same time becoming strongly basophil (fig. 3, en.c). These 

 cells contain but little yolk, otherwise being identical with those 

 arising from the primitive blood-cells. Endothelium cells in 

 various parts of the body become thickened and more basophil, 

 bulging out into the loose mesenchyme. Often free lymphoid 

 wander-cells are seen lying in contact with these endothelia. 

 Lymphoid wander-cells occur in many places in the mesenchyme : 

 in the head mesenchyme, in the interstitial spaces of the meso- 

 nephros, around the gut. Here they arise from the indifferent 

 mesenchyme cells by a withdrawal of their processes and an 

 increase in the basophilia of their cytoplasm, the nuclei changing 

 but little. It is likely that the cells resembling primitive blood- 

 cells which were seen in the loose mesenchyme of the earlier 

 stages (4 to 5 mm.) have gone over into lymphoid wander-cells 

 or extravascular large lymphocytes. 



Although white cells do arise from these other sources, both 

 within and outside the vessels, they are not, as Maximow ('10) 

 claims they are in Rana temporaria, the sole source of white 

 blood-cells, for continuous series of transitions from yolk-laden 

 primitive blood-cells to yolk-free, and also yolk-laden, large 

 lymphocytes are found and these yolk-laden large lymphocytes 

 cannot have been derived from either the endocardium cells, 

 which are poor in yolk, or from the now almost yolk-free endo- 

 thelium and mesenchyme cells. 



At . this stage, 5- to 6-mm. body length, the liver anlage is 

 invaded by blood-vessels. These narrow vessels contain but 

 few blood-cells, so the hematopoietic function of this organ is 

 not yet developed at this period. 



