EHYTHHOPOIESIS IN YOLK-SAC OF PIG EMBRYO 295 



consisted in tho stoj^pafio of the embryonic circulation by means 

 of anaesthetics. Stockard's observations led him to conclude 

 that in the Fundulus embryos investigated (uj) to 20 days) the 

 entlothelium plays no haemogenic role. In the pig embryo, on 

 the contrary, the data leaves no escape from the opposite con- 

 clusion, a conclusion arrived at also by many investigators of 

 \'ari()us embryo forms, [e.g., certain chelonians, Jordan and 

 Flijipin (13)]. This conclusion is supported by the further 

 imj^ortant fact that the endothelium of the sinusoids of the liver 

 and of the glomeridar capillaries of the mesonephroi also produce 

 haemoblasts. 



The sole alternative interpretation that has an^' appearance 

 of plausibility respecting the haemoblasts of the yolk-sac vessels 

 here described as separating from the endothelium, is that they 

 have become pressed against the wall and thus modified in shape 

 and caused to adhere intimately to the endothelium, so as to 

 stimulate endothelial continuity and derivation. This sugges- 

 tion is rendered inapplicable by 1) the possibility of tracing a 

 complete series of transition stages between a true endothelial 

 cell, through intermediate haemoblast stages, to a free haemo- 

 blast; 2) the possibility of tracing a similar series through to 

 multinucleated giant cells; 3) the fact that such haemoblasts in 

 apparent continuity with the endothelium are cjuite as abundant 

 in essentially empty vessels as in vessels crowded with blood 

 cells, where alone an adecjuate factor of pressure would seem to 

 prevail, and 4) that haemoblasts, though apparently naked 

 cells, do not in general exhibit adhesive properties except among 

 themselves. 



b) Giant celh. The deri\'ation and the morphologic and ca'- 

 tologic variations of the giant cells are clear, as described above. 

 These cells are simply modified haemoblasts, capable of under- 

 going a similar differentiation into giant erythroblasts, and ai)ixir- 

 ently ultimately diffcn-entiating normoblasts intracellularly. This 

 last point may be thought perhaps to remain somewhat doubt- 

 ful, and e\'en if the interpretation is accepted, the significance 

 and economy of this process — for it is clearly not essential, since 

 it is not the exclusive method for yolk-sac haemopoiesis still 

 remains obscui'e. 



