580 CHARLES R. STOCKARD 



seen anything to indicate that a fully formed, erythrocyte was 

 capable of automatic migration. Yolk-sac blood islands of all 

 ages have been examined in great numbers, but never has an 

 erythrocyte appeared wandering away from such an island into 

 neighboring regions. This fact is most important in the study 

 of the blood-islands in the non-circulating specimens. 



When the slightest flow does exist for any length of time, 

 there is a definite tendency, as mentioned above, for the blood 

 to accumulate in certain sinuses and vessels. The positions of 

 accumulation vary somewhat with the stages at which the 

 circulation ceased, as well as the manner of stoppage of the flow, 

 whether it was gradual or sudden. 



When the circulation stops during early stages, there is a great 

 accumulation or massing of the blood over almost the entire 

 ventral surface of the yolk. In other words, there is a hemorrhage 

 or bleeding into these spaces or vessels until no more blood is 

 left in other regions of the embryo, the heart gradually becomes 

 empty of corpuscles and no longer passes them along. The 

 packing of the yolk vessels probably clogs or blocks the circu- 

 lation so that it ceases. Again, the circulation may stop more 

 suddenly and the venous end of the heart or the entire heart may 

 be seen packed with corpuscles while the vessels immediately 

 entering and leaving it are comparatively or entirely empty. 

 In older embryos there is the tendency to accumulate red cells 

 in the vessels of the head so that brilliant red clots are frequently 

 seen in these positions. 



In all cases it is interesting in these individuals in which the 

 circulation has ceased at one or another period in development, 

 and doubtless for different reasons in different specimens, to 

 observe the way in which the blood sooner or later accumulates 

 in one or another vascular space and does not remain uniformly 

 distributed throughout the vascular system. Only when the 

 heart is suddenly stopped and the blood quickly fixed by some 

 strong killing fluid does one get a good pattern of the vascular 

 system loaded with corpuscles throughout most of its extent. 

 In rare cases, three during the present summer in some hundreds 

 of embryos examined, will a specimen without a circulation at 



