DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 549 



as would be expected, yet the solid string-like heart present in 

 many such specimens is also covered with pigment though 

 it of course is entirely empty of plasma. In this last case, how- 

 ever, the string-like heart actually stretches as an axis through 

 the pericardial space which is distended with fluid. The cells 

 arrange themselves around the wall of the pericardium and on 

 reaching the venous end of the heart migrate along it and so cover 

 the heart string or tube in their effort to come in close proximity 

 to the plasma. The distended condition of the pericardium 

 may in this way account for the pigment arrangement along the 

 heart in the cases with experimentally arrested circulation. 



The normal heart is constantly pumping the plasma through 

 itself, yet pigment cells are never present in its wall since they 

 are all arranged along the vessels of the yolk-sac. In non-cir- 

 culating cases many vessels form on the yolk-sac and some be- 

 come quite well developed, while others actually surround the 

 blood corpuscles of the yolk islands. Such vessels are at times 

 covered with pigment but probably through accident as the 

 pigment is irregularly scattered over the entire yolk-sac. Yet 

 the pigment cells on such vessels never arrange themselves in 

 the definite sheath-like fashion characteristic of the vessel pig- 

 ment in the normal embryo. 



Figure 18 illustrates the lack of arrangement of the chromato- 

 phores on the yolk-sac of a 16 day embryo without a circulation; 

 compare figures 15 and 17 of pigment in the normal embryo. 

 Figure 35, see beyond, also illustrates in a striking way the 

 irregular grouping of black chromatophores in the neighborhood 

 of a collection of stagnant blood islands in an embryo of 14 days 

 that never had a circulation of its blood. 



All of these reactions cause one to wonder what is the actual 

 function of the pigment cells upon the yolk-sac. The entire 

 egg is rather transparent and their function might be to protect 

 the vessels from the light, yet the vessels are never completely 

 covered and the development of the eggs in the dark is normal 

 but not in any way supernormal. 



The pigment cells form such a complete sheath about the 

 vessels in some cases that one might be led to imagine that their 



