270 CHARLES R. STOCKARD 



to be in any way related except that all are of mesenchymal 

 origin. The chromatophores as before mentioned, often come to 

 lie along the walls of the blood vessels. 



The early yolk-sac is non-vascular, the blood masses being 

 completely uncovered by endotheUum. Later endothelial walls 

 are formed around the blood cell masses and a vascular net- 

 work is estabhshed in the yolk-sac of the normal embryo though 

 poorly formed in the individuals without a circulation. All of 

 these yolk vessels seem to arise by arrangement of wandering 

 mesenchimal cells. Certain of these cells elongate and group 

 themselves in such a way as to form vessel tubes. After the 

 vessels are formed they may then be seen to send off buds and 

 sprouts in the manner Clark ('09) has described in amphibians. 

 The difference between the cells giving rise to the vascular endothe- 

 lium and those forming the blood cells is not distinguishable in 

 early stages. Yet after considerable study and careful observa- 

 tion, nothing has been observed that would indicate that these 

 vascular endothehal cells possess the power to change into the 

 blood cell type, nor is there smy evidence to indicate that cells hav- 

 ing once assumed even the earliest blood cell type are capable of 

 metamorphosis to form endothehal cells. It is impossible to 

 state emphatically that the vascular endothehum of the yolk- 

 sac in all Teleosts arises in the same way as that described here 

 for Fundulus embryos. But any one famiUar with the very 

 complex yolk circulation of the trout family, in the light of the 

 above knowledge is scarcely justified in assuming, that this 

 network of vessels is completely derived from outgrowths from 

 the aorta and cardinal veins within the embryo as Sobotta 



Fig. 34 Section through the yolk-sac of an embryo seventy-two hours old, 

 without a blood circulation. A group of cells forming a blood island are distin- 

 guished by a slight condensation of cytoplasm about their nuclei; Experiment A, 

 1913; Ec, the ectoderm several cells thick; Bi, the cells of the blood island; Yk, 

 granular yolk. 



Fig. 35 Young erythroblasts just isolating themselves in another island 

 on same yolk as figure 34. Compare the early blood cells with those of figures 

 31 and 32, in the intermediate cell mass. 



Figs. 36 to 39 Illustrate the progressive steps in the development of the 

 network of yolk-sac blood islands. 



