ORIGIN OF BLOOD AND ENDOTHELIUM 237 



heart-beat or circulation to be seen with the high power microscope. 

 At seventy-five hours the pigment particles are just beginning 

 to show in the chromatophores. A well formed vesicle is clearly- 

 seen at the posterior end of the embryo from forty-eight to seven- 

 ty-two hours and older. This is the so-called Kupffer's vesicle, 

 and it, like the pericardium, becomes greatly distended by an 

 accumulation of plasma in those individuals which have no cir- 

 culation. 



Embryos with fourteen somites may still have no heart beat 

 and no circulation of the blood. Any later than this, how- 

 ever, all normal embryos establish a heart-beat and a circulation 

 of colorless fluid, there being no blood cells present in the initial 

 circulating medium. Very soon after the circulation is estab- 

 lished at first a few but quickly many blood cells are added to 

 the stream. This is merely an abbreviated summary of the 

 development of the Fundulus embryo up to the time of the 

 establishment of the heart-beat and circulation, but as stated 

 above, the rate is variable and it often happens that an embryo 

 of seventy-two hours has already established a vigorous circu- 

 lation and the plasma is loaded with well formed blood cells. 



2. History of experimental embryos to the time when a circulation 



should begin 



In the experimental embryos development proceeds more 

 slowly than in the normal. The plasma which should circulate 

 in the vessels accmnulates in the sinuses over the yolk and finally 

 seems to collect in great amount in the pericardium, the lateral 

 coelomic cavities and in Kupffer's vesicle, so that these spaces 

 become hugely distended and appear as great sacs or vesicles of 

 colorless watery fluid. The excessive presence of this fluid in 

 the pericardium seems to exert a mechanical effect which tends to 

 separate the head of the embryo an unusually great distance from 

 the yolk mass and thus stretches the heart out into a long string- 

 hke cord, passing from the embryo to the surface of the yolk. 



This pushing away of the head from the yolk is very well 

 indicated in figures 15 to 20, which show various types of hearts 

 found in these embryos during later stages. The stretching or 



