ORIGIN OF VASCULAR TISSUES 107 



a sufficient negative pressure to have moved these erythrocytes 

 to their present location fi'om another region of the embryo. 



Numerous other cases were observed in which here and there 

 a captured mesenchyme cell had probably changed into an eryth- 

 rocj'te. It is likely that the results of chemical treatment tell 

 us little more concerning the potentialities of the fully developed 

 liver than they tell us of the potentialities of spleen and bone- 

 marrow. 



Jj.. Ova subjected to low temperatures 



Another means of preventing blood-circulation in Fundulus is 

 to subject the ovum to low temperatures at a very early stage 

 of development. This method of developmental arrest is not 

 new. Four-celled ova were kept at about 4°C. for periods of 

 time ranging from twenty-four to seventy- two hours. Develop- 

 ment under these conditions proceeded rather slowly. Embryos 

 often developed without circulation. In all probability the low- 

 ering of the temperature causes a blastolysis (Werber, 79). The 

 yolk-sac in such embryos is often greatly shrunken away from the 

 chorion, and evidence of egg-extrusions can often be found in the 

 subchorionic space. Embryos so treated often fail to develop 

 hearts. It may be that the region later to produce the heart- 

 anlage in such cases becomes blastolyzed. At any rate, these 

 embiyos devoid of hearts are of interest owing to the fact that 

 their anterior mesenchyme is sometimes hematopoetic. I have 

 obtained erythrocytes in such embryos in positions similar to 

 those shown in figures 50, 54, and 55. 



5. Cardiectomized embryos 



Removal of the heart prior to heart-pulsation is the ideal 

 means by which one may assure himself that no elusive circula- 

 tion will falsify the picture obtained in the study of blood-devel- 

 opment. The heart region may be destroyed even before a 

 heart-anlage can be found. If this be done the question of pas- 

 sive movement of the blood-elements can be eliminated. The 

 operated embryos may be placed in any position with reference 



