DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 131 



d. Relationship of chromalophores to blood and endothelial 

 cells. There has been much discussion in the literature regarding 

 the relationship of the chromatophores to blood corpuscles and 

 to endothelial cells. The actual relationship of these cells is 

 clearly brought out by a careful study of the living yolk-sac in 

 Fundulus. The cells are completely different and their struc- 

 tures when once established are consistent in their particular type. 



The black chromatophores, the brown chromatophores, the 

 endothelial cells and blood corpuslces are all derived from mesen- 

 chymal cells which wander away mainly from the caudal region 

 of the embryo during early stages of development. These cells 

 come to lie in the primary segmentation cavity of the yolk-sac 

 beneath the ectoderm and over the periblast syncytium. The 

 mesenchymal cells very soon begin to show certain differential 

 characters in structure and behavior. When certain ones of the 

 cells incline in a definite direction their development progresses 

 continuously along this line. 



Observations on the normal living embryos and comparison 

 with those individuals without a yolk-sac circulation lead one to 

 conclude as follows regarding the wandering mesenchymal cells. 

 At the time these cells leave the embryo proper to wander over 

 the yolk, differentiation has proceeded to some extent in the 

 embryo and the mesenchyme cells are probably somewhat limited 

 hi their potentialities. Certain of them are derived from the 

 same portion of mesenchyme that gives rise to the intermediate 

 cell mass or future red blood cell forming mass within the embryo. 

 This mass is located towards the caudal end of the embryo and 

 the wandering cells derived from it finally come to lie on the pos- 

 terior and ventral surfaces of the yolk-sac and form islands of red 

 blood corpuscles. Few if any of these cells reach the anterior 

 regions of the yolk-sac before the circulation of the blood begins. 

 In embryos that never have a circulation the blood islands lie 

 beneath the tail of the embryo and on the ventral yolk surface. 

 The future endothelial cells wander out from the caudal end and 

 side of the embryonic body and finally line up to form vessels 

 in a manner to be described beyond. The pigment cells also 

 wander out from the lateral regions and differentiate into 

 chromatophores of either the black or brown variety. 



