ORIGIN OF VASCULAR TISSUES 89 



(55) have shown that in hybrid embryos of the teleosts the vi- 

 tahty of the embryo is often so low that it is never able to de- 

 velop endothelium beyond the stage of the independent anlagen. 

 The embryo from which oar figures 13 to 15 (pp. 94-96) were 

 taken is of unusual interest, both because of the condition of its 

 endothelium and the position of its blood-cells. Later, Newman^ 

 states : 



Among the most interesting anomalous conditions seen in these 

 hybrids are the various disturbances in relation to the parts of the 

 vitelline and systemic circulation. The heart and its main vessels 

 frequently appear disjoined from the body and exhibit an independence 

 in differentiation and an automaticity truly striking. Many problems 

 might be cleared up by a study of these conditions. 



Incidentally it might be mentioned that my own results ob- 

 tained from chemical treatment confirm the observations of 

 other observers concerning the origin of endothelium under these 

 conditions. 



6. Observations on living teleost material 



In the yolk-sac of the chemically treated teleost embryo from 

 which figures 61 and 62 were made, mesenchyme cells were fol- 

 lowed in their active migration during their process of transfor- 

 mation into endothelium. Individual cells in some cases aligned 

 themselves into solid columns which later became hollow. 

 Among the more rounded endothelial anlagen a case was ob- 

 served in which a rather confused group of young blood-cells and 

 intermingled mesenchyme-cells arranged themselves into a vesi- 

 cle containing blood-cells. Anastomoses between already formed 

 vascular cavities were seen in some cases to be a result of an 

 actual sprouting of the endothelium itself. 



In the cardiectomized embryo from which figure 82 was made, 

 the heart was removed while it was a very slender tube. It had 

 never pulsated. The living mesenchyme cells were later fol- 

 lowed on the yolk of this embryo. They were migrating from 

 the posterior body-region to the posterior portion of the yolk-sac. 



2 Newman, H. H., Proc. Am. Soc. Zool., 1915. 



