128 CHARLES R. STOCKARD 



During the early stages, up to the beginning of the fourth day, 

 the cells wander in amoeboid fashion much the same as in ordi- 

 nary specimens. In other words, at this tune the condition is the 

 same in all embryos since the blood has not begun to circulate 

 in any. At about 72 hours the blood circulation begins in the 

 normal embryos and the pigment cells seem to be attracted to 

 the vessel walls, as already pointed out. If the circulation does 

 not begin at this age, the plasma accumulates in various spaces, 

 chiefly the pericardial sac and Kupffer's vesicle at the caudal 

 end of the embryo. The excessive accumulation of plasma in 

 these spaces causes them to be in many cases hugely distended. 

 The heart in such specimens also becomes a sacular structure 

 filled with plasma which it is unable to pump on account of one 

 or another deficiency in the vascular system. 



Large numbers of chromatophores of both types tend to aggre- 

 gate about these plasma filled spaces and partially cover their 

 walls. The spaces are thus rendered more conspicuous. In 

 some specimens this coating of the distended plasma sacs by pig- 

 ment cells is most remarkable, but such an arrangement is not 

 invariable and in a number of individuals the pigment cells are 

 irregularly scattered over the yolk-sac with no recognizable 

 pattern or system. 



The heart of embryos in which there is no blood circulation is 

 almost without exception covered with chromatophores. These 

 cells often form a perfect sheath about such hearts whether the 

 heart is a plasma filled sac or a mere string. The patterns of 

 these arrangements are illustrated by numerous figures, partic- 

 ularly figures 15 to 20 in the previous paper. 



A point of much interest in this connection is the fact that the 

 heart of the normal embryo is entirely free of pigment cells. 



The behavior of the chromatophores of the yolk-sac in normal 

 individuals where they tend so decidedly to arrange themselves 

 along the blood vessel walls along with their affinity for the 

 plasma filled spaces in the non-circulating condition would seem 

 to indicate that the chromatophore was attracted by the plasma 

 itself, or some element which it contains. The distended plasma 

 filled heart in the non-circulating cases is covered with pigment 



