96 CHARLES R. STOCKARD 



come erythrocytes filled with haemoglobin. Typical vascular 

 endothelium completely surrounds the erythrocytes . which 

 instead of being swept away as usual by the circulating current 

 remain in their place of origin. All of the early blood forming 

 cells of this intermediate mass give rise only to erythroblasts. 



6. Contrary to the opinion of most recent observers on blood 

 development in Teleosts, the Fundulus embryos both with and 

 without a circulation possess blood islands on the posterior and 

 ventral portions of the yolk-sac. These blood islands are formed 

 by wandering mesenchymal cells which migrate out from the 

 posterior region of the embryo. They represent all that remains 

 of the peripheral yolk-sac mesoderm in the Teleosts and probably 

 wander way from mesoderm related to that of the intermediate 

 cell mass. The intermediate cell mass may possibly represent 

 the bulk of the peripheral mesoderm which is here included within 

 the embryonic body, while in other meroblastic eggs it is spread 

 out over the yolk. The only mesodermal portion of the yolk- 

 sac in Fundulus is made up of the disconnected wandering 

 mesenchyme cells some of which group themselves to form the 

 blood islands, while others give rise to the yolk vessel endothelium, 

 and still other wandering cells develop into the chromatophores. 



7. The non-circulating red-blood corpuscles within the em- 

 bryo remain in a fully developed condition for eight or ten days 

 and then undergo degeneration. In an old embryo of sixteen 

 days it is sometimes found that very few of the corpuscles in the 

 intermediate mass are still present and these are degenerate. The 

 vascular endothelium has been lost and numerous mesenchymal 

 cells have wandered in and lie among the corpuscles. 



On the yolk-sac the corpuscles no doubt have a better oxygen 

 supply and here they maintain their color longer but finally also 

 present a degenerate appearance with small densely staining 

 nuclei and cell bodies much reduced in size. 



8. Vascular endothelium arises in loco in many parts of the 

 embryonic body in which blood cell anlagen are not present. 

 This endothelium is in all cases utterly incapable of giving rise 

 to any type of blood cell. This incapacity cannot be attributed 

 to the abnormal condition of the embryo as true blood cell anla- 

 gen in the same specimen produce blood corpuscles in abundance. 



