578 CHARLES R, STOCK ARD 



separated so as to allow free respiration. The egg membrane is 

 covered with long hair-like filaments which become entangled 

 with those of neighboring eggs and in this way masses become 

 closely packed. The central eggs of such a mass develop in a 

 poor atmosphere and go more slowly than their neighbors on 

 the outside of the mass. These centrally located eggs show many 

 abnormal and arrested conditions of much the same type as may 

 be obtained by treating the eggs w^ith various injurious solutions. 



The changeable states in the circulation offer many pitfalls 

 for one attempting to determine the sites of origin of blood cells 

 in the non-circulating embryqs. Old embryos are seen in which 

 there are beautiful blood islands on the yolk-sac and great clots 

 of blood in the head or other unusual position. The heart is 

 very frequently completely loaded with corpuscles, and yet 

 there is not the slightest movement of the blood cells or any 

 sign of a circulation at this time. The heart itself may be pul- 

 sating feebly or even practically stopped. 



Another source of blood movement which is slight, yet to be 

 guarded against, is that due to the muscular twitching of the 

 embryo's body. This movement may frequently serve to push 

 cells from the intermediate cell mass out on to the yolk-sac, but 

 usually by way of the vessels. These dangers are to be taken 

 seriously in experiments of this kind. Since one is able to be 

 absolutely certain that the blood never circulates in a great num- 

 ber of embryos, only such embryos should be considered in a 

 study of blood origin. During a study of this exact problem now 

 extending over four spawning seasons, I have seen blood in 

 almost every conceivable position in embryos without a circu- 

 lation at the time of the observation. The accumulation of blood 

 is more frequent in certain positions and regions than in others. 

 The venous end of the heart is a most common place for a clot, 

 the sides of the head, the large vessels of the yolk just lateral 

 to the body, and various places on the anterior and lateral yolk 

 surfaces. 



When, however, the experimenter collects a number of embryos 

 that have really never experienced the slightest flow of their blood, 

 ihe case is very definite. No blood clots ever occur in regions other 



