ORIGIN OF BLOOD AND ENDOTHELIUM 43 



The yolk-sac of the bony fish is pecuHar in this connection. 

 In most meroblastic embryos there is a definite mesodermic 

 layer or membrane between the ectoderm and entoderm of 

 the yolk-sac, and it is in this mesodermal layer that the blood 

 islands arise. When one examines the yolk-sac of the Teleost 

 embryo, the mesodermic layer is found to be largely, if not en- 

 tirely, absent. Thus, the ectoderm Hes directly over the yolk 

 periblast which may be considered to represent the primary 

 entoderm. Between these two layers many long spindle-shaped 

 mesenchjrmal cells are noticed on careful examination, but these 

 cells in the specimens examined are never arranged in a definite 

 continuous layer. 



Goodall ('07) has recently stated that in the sheep embryo, 

 the yolk-sac mesenchyme is not to be considered a continuous 

 layer, but consists merely of diffusely scattered wandering mesen- 

 chymal cells. These mesenchymal cells in the sheep as in the 

 fish finally collect into groups and such groups ultimately give 

 rise to the blood islands. In the fish it would seem as though 

 the entire ventral or yolk-sac mesoderm, the chief source of 

 blood formation, had been in its phylogenetic development in- 

 corporated or drawn into the body of the embryo as the inter- 

 mediate cell mass, and only a few cells lag behind or later wander 

 out to form the collections of mesenchymal cells upon the yolk. 

 Ontogenetically there is no longer any indication of a mechanical 

 drawing-in process but the wandering out of cells may be readily 

 observed. It is also easily conceivable that this condition proba- 

 bly differs in different species of Teleosts. Therefore, some species 

 may really form no blood cells in the yolk-sac, while again others 

 might have an almost complete mesenchymal layer in the sac 

 and in such a case would probably give a typical blood island 

 arrangement. Whereas, an intermediate condition would be well 

 represented in the species of Fundulus here studied in which 

 there are numerous disconnected wandering cells later grouping 

 themselves to form the blood islands on the yolk-sac. 



The appearance of the wandering cells as they radiate out 

 from the caudal end of the embryo on to the yolk-sac is strikingly 

 similar to that shown by the cells wandering away from the cen- 



