DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 135 



A slightly older embryo shows a still more definite alignment 

 of the mesenchymal cells and still later presents the appearance 

 of cellular strings or cords as illustrated in an embryo of 67 hours 

 by figure 22. Here the wandering mesenchyme cells have differ- 

 entiated to such an extent that they are readily distinguishable 

 as black and red chromatophores and elongate endothelial cells. 



22 



Fig. 22 A sketch of a 67 hour embryo showing the stage in the origin of yolk 

 vessels in which the mesenchymal cells have a linear arrangement. Early black 

 and brown chromatophores are also shown in the yolk-sac. Od, oil drops. 



Early erythroblasts are also seen on the caudal region of the yolk- 

 sac in such an embryo, but are not shown in the aspect here 

 illustrated. 



The endothelial cells are strung out in various directions and 

 several linear groups are more or less isolated from the rest. The 

 string to be the future large vitelline vessel is not clearly continu- 

 ous posteriorly, but anteriorly it is well outlined extending to- 

 wards the venous end of the now forming heart which has not yet 



