236 CHARLES R. STOCKARD 



sixteen days old, the normal embryo hatches and becomes free 

 swimming at about twelve days. 



As mentioned above, a similar series of normal embryos have 

 been prepared and used for comparison with these non-circulating 

 individuals. 



In order to be certain of the final developmental product of 

 the blood cells of the fish, mmierous smears have been made 

 from various tissues and from heart blood taken from the adult 

 Fundulus. In these smears one finds the various types of white 

 blood cells and the ordinary red blood corpuscles of Teleosts. 



THE STUDY OF LIVING EMBRYOS WITH AND WITHOUT 

 THE CIRCULATION OF THE BLOOD 



1. Normal development up to the establishment of a circulation 



The rate of development of Fundulus embryos is very variable, 

 differing at different periods of the breeding season, and also 

 differing in groups of eggs from different individuals. 



When twenty-four hours old, the germ ring has descended 

 almost to the equator in the most rapidh^ developing individuals. 

 In others the ring is only one-third way over the yolk sphere. 

 The embryonic shield and the first line indicating the position 

 of the embryo's body is now to be made out. At forty-eight 

 hours, the yolk sphere is complete^ covered by the ectoderm, the 

 embryonic body is well shown, with the optic knobs projecting 

 prominently and several somites easily distinguishable. The 

 heart has not begun to pulsate and no blood cells or blood anlage 

 are distinguishable in the living specimen. Very soon after this 

 time, or at least by sixty-eight hours, there are about ten somites 

 present and collections of cells on the yolk-sac are the first indica- 

 tion of blood islands. No pigment cells have formed up to sixty 

 hours. 



At seventy-one hours pigment cells are recognizable but the 

 blood islands are not j^et colored and are sparsely arranged over 

 all the yolk region except the anterior haK. Near the lateral 

 borders of the embryo and on the posterior yolk surface the 

 islands are most abundant. At this time there is still no visible 



