20 CHARLES R. STOCKARD 



on the yolk-sac of a normal embrv'o. While in embryos treated 

 with KCl in which there was no circulation, the pigment cells 

 failed to asume any definite pattern. They remained more or 

 less indefinitely scattered over the surface of the yolk and the 

 body of the embrj'o in no way tending to align themselves along 

 the vessel walls. From this fact, Loeb concluded that it was 

 probably due to some chemotactic reaction that the pigment 

 cells lined up along the blood vessels when the blood began to 

 circulate and the attracting substance was possibly the oxygen 

 contained within the blood corpuscles. Observing the various 

 indi\'iduals without a circulation which we shall here consider, 

 it will be seen that the pigment cells have a strong tendency 

 to migrate to any cavity filled with plasma or fluid, and it is 

 not probable that this plasma or fluid contains any more oxj^gen 

 than is present in the other portions of the yolk-sac or body. 

 It would, therefore, seem more hkely that some constituent of 

 the plasma itself and not the oxygen contained within the blood 

 cells was the stimulating principle which caused the migration 

 of the pigment cells to the vessel walls. 



5. The eight- and ten-day embryos 



The next group of figures illustrates the advanced condition 

 which the embryo has reached by the eighth day. Figure 7 

 represents a normal Fundulus embrj-o of this age. The body 



Fig. 5 An embryo eight days old, without a circulation; the heart, ht, poorly 

 developed, beats feebly twenty-eight times per minute, about one-quarter the 

 usual rate; Pc, pericardium greatly distended with fluid; from a "1.5 cc. alcohol 

 solution." 



Fig. 6 Eight-day embryo without a circulation, ht; the heart dilated with 

 plasma pulsates ninety-five times per minute; RCh, the red chromatophores 

 beautifully expanded, but no vessels present on the j'olk. Coe, the lateral coelo- 

 mic cavity dilated with fluid; icm, the intermediate cell mass now a great string 

 of red blood corpuscles. 



Fig. 7 A normal eight-day embrj-o, the heart, ht; pulsating rapidly and the 

 network of yolk vessels mapped out by the chromatophores. 



Figs. 8 and 9 Two eight-day embryos without blood circulation; chromato- 

 phores unexpanded but collected on the heart, ht; the normal heart has no pigment 

 cells on it; Pc, the dilated pericardium; icm, median mass of erj'throblasts: 

 CV, cardinal vein containing erythroblasts. 



