ORIGIN OF BLOOD AND ENDOTHELIUM 253 



7. Develop7?ie7it of the yolk-sac blood islands in life 



The blood islands in the living embryos, as was mentioned 

 before, are quite difficult to see in the early stages. But a few 

 hours before the heart begins to pulsate and the circulation be- 

 becomes established they are very evident in the posterior ventral 

 yolk regions. The arrangement of the blood islands display 

 various patterns in different individuals in some being incon- 

 spicuous while in others an extensive network is present. These 

 blood islands probably arise largely from wandering mesenchymal 

 cells since the yolk-sac of the Fundulus embryo consists at first 

 only of the yolk periblast with the ectoderm immediately above it. 

 There is no true mesodermal layer to the yolk-sac and this 

 mesenchymal blood formation on the yolk can, in all cases, be 

 traced to the early wandering cells. 



In figure 21 the posterior end of an embryo of ninety-six hours 

 is shown. The Kupffer's vesicle, Kv, is dilated and pigment 

 cells have accumulated around it. Immediately posterior to 

 this are a number of blood islands indicated by stippling. 



Figure 22 shows an embryo eight days old with the posterior 

 ventral surface of the yolk well covered with blood cells. Eryth- 

 rocytes are also seen in the intermediate cell mass. The blood 

 in this embryo has never circulated and one can scarcely conceive 

 that the blood islands on the extreme ventral surface of the 

 yolk are due to the crowding out or pushing away of cells from 

 the intermediate cell mass within the embryo. These cells are 

 rather to be regarded as true yolk-sac blood islands which have 

 arisen from early wandering mesenchymal cells probably in the 

 beginning derived from same source as the intermediate cell mass. 



Figures 23 and 24 show the caudal ends of two normal embryos 

 of seventy-two hours. In these the heart has not begun to 

 pulsate nor the blood to circulate, yet a distinct group of eryth- 

 roblasts or early blood cells are seen already arranged on the 

 yolk-sac in this posterior region. 



In figure 24, it would look as though these cells had wandered 

 out from and grouped themselves around the tail end of the 

 embryo. At this period, seventy-two hours, the intermediate 

 cell mass within the embryo is not visible in life. 



