DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 591 



The movements of these extremely numerous cells and their 

 changes of position may be readily followed with a high magni- 

 fication. In embryos of about 60 hours, still some time before 

 the heart begins to beat or the blood to flow, four clearly distinct 

 types of cells may be recognized among these originally similar 

 mesenchymal cells, and the further history of the four types 

 has been completely traced. 



2. The amoeboid cells with conical pseudopod-like processes 

 shortly after 60 hours begin to show an accumulation of pigment 

 granules within their cytoplasm. Just at this time they are seen 

 to be of two distinct varieties, one depositing a black and the 

 other a brownish red pigment. 



The black chromatophore increases rapidly in size and by the 

 end of the third day becomes an enormous amoeboid body 

 wandering over the yolk. These cells are attracted to the walls 

 of blood vessels and plasma filled spaces, such as the pericardial 

 cavity becomes in specimens without a blood circulation. 

 When the embryo is five days old the chromatophores are 

 abundantly arranged along the walls of the vitelline vessels, 

 but the pigmented cells are distinctly separate. After this 

 time neighboring cells begin to fuse along their adjacent borders 

 and large pigment syncytia are formed which completely surround 

 and ensheath the vessels. A single syncytium is often of con- 

 siderable extent, as shown in figure 15. 



The brown chromatophores have a somewhat different history. 

 They never become so massive as the black, and their processes 

 are more delicate and graceful in appearance. Yet these cells 

 also attain a large_^ size and in embryos of 72 hours are scattered 

 over the entire yolk-surface. After the third day when the blood 

 begins to flow in the yolk vessels, the brown chromatophores 

 likewise become attracted to the vessel wall. These exquisitely 

 branched cells apply themselves to the wall of the vessel and may 

 often completely surround it, as shown in figure 17. This type 

 of chromatophore, however, always maintains its cellular indi- 

 viduality and never fuses with other cells to form a syncytium 

 as is the case with the black type. 



The function of the chromatophores on the yolk-sac is most 

 difficult to decide, but one thing is certain, they never, become 



