146 CHARLES R. STOCKARD 



within the embryonic body. The differentiation of this mesoderm 

 within the embryo is much the same as that of the peripheral 

 ventral mesoderm in the yolk-sac of other groups. Thus in the 

 bony-fish the great bulk of blood formation takes place within 

 the so-called intermediate cell mass, the probable homologue 

 of a portion of the yolk-sac mesoderm of other vertebrates. 



The intermediate cell mass first described by Oellacher ('73) 

 and later fully studied by Ziegler ('87), Swaen and Brachet ('99, 

 '01), and numerous others, is derived from the primary lateral 

 plate mesoderm, separating away from the median border of this 

 plate. The cell masses of the two sides remain apart in some 

 species and form the future cardinal veins and red blood cor- 

 puscles, while in others the two masses unite in the median line 

 to form the conjoined cardinals or stem vein, which is loaded 

 with the primitive erythroblasts the red blood anlage. 



All recent workers on the development of the blood in the 

 bony-fish have considered this intra-embryonic blood formation 

 as being the only source of blood cells in these animals. Several 

 authors, however, Swaen and Brachet among them, have recog- 

 nized that the cells of the stem vein may become so packed and 

 crowded within the embryo that masses of them are directly 

 pushed out laterally upon the yolk. They have also thought 

 it possible that a very few cells might wander upon the yolk- 

 sac and there form blood, but this has been considered question- 

 able in all cases. No one has recognized the actual occurrence 

 of blood islands upon the teleostean yolk-sac. Even Wencke- 

 bach in his study of living embryos, although he made so many 

 important observations on the formation of the periblast and 

 yolk vessels, entirely overlooked the early or primitive yolk-sac 

 blood cells. This is probably due to the fact that he studied 

 only normal embryos. In embryos without a circulation of the 

 blood one observes the yolk islands much more readily as the 

 cells finally become filled with haemoglobin and present a bright 

 red color. When they are once located in these experimental 

 embryos it becomes much easier to trace them in the younger 

 normal individuals, and finally the observer readily locates these 

 cells and may follow completely their migrations and association 

 to form the yolk-sac blood islands. 



