16 CHARLES R. STOCKARD 



cally similar to that just described. The islands on the yolk- 

 sac are often very far distant from the embryonic body as is 

 readily seen by reference to figures 21 to 24. There is no doubt 

 that wandering cells migrate out on to the yolk surface at a very 

 early period and here give rise to blood islands comparable to 

 those formed in other meroblastic embryos. It must be recog- 

 nized, as I shall bring out in a consideration of the microscopic 

 study of these embryos, that the yolk-sac of the fish is not en- 

 tirely comparable to that of all other vertebrate types, yet there 

 are many observations on the early living embrj^os which have 

 convinced me that mesenchymal cells do wander from the embryo 

 to various parts of the yolk-sac. These cells occupy a position 

 between the ectoderm and the periblast (periblastic endoderm ?) 

 just as the peripheral mesoderm would in other yolk-sacs. 

 Some of the wandering cells are future pigment cells of either 

 the red or black variety to be mentioned later, others future 

 endothelial cells, but many at least are to give rise to future 

 blood cells. 



Therefore, in embryos at about the beginning of the circulation 

 one finds two distinct blood regions: The major region and most 

 evident is the intermediate cell mass of former investigators, 

 and the second position in which the blood cells are seen is the 

 yolk-sac blood islands. The earliest yolk-sac blood islands are 

 very easily overlooked. The writer had examined these embrj^os 

 in great numbers and studied them for sometimes before finally 

 discovering the existence of the early islands. With a high 

 power single objective binocular, however, after their location is 

 known the observer is readily able to see the nuclei of these 

 cells in the posterior region of the yolk-sac, and they may then 

 be followed from time to time. After these observations there 

 is finally no doubt that blood islands do form on the yolk-sac 

 of the Teleost embryo but these islands are probably to be re- 

 garded as disconnected portions of the intermediate cell mass 

 or blood anlage. 



It is freely admitted that this yolk-sac island blood formation 

 may not take place in all Teleosts. The early wandering cells 

 that here give rise to blood islands may in reality be compared 



