DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 127 



Finally, in older embryos the cell body often surrounds a 

 vessel, as shown in figure 17, but the processes persist and pro- 

 ject from it in all directions, forming a striking contrast to the 

 more or less smooth outlines f nally assumed by the black cells, 

 figures 14 and 15, as they surround the vessels. 



The brown chromatophores do not group themselves together 

 or form a syncytial mass as the black pigment cells are prone to 

 do. They remain individually separated and many really 

 never become associated with vessel walls, but lie scattered on 

 the yolk surface. 



In early embryos, from 72 to 90 hours, the brown pigment 

 cells may sometimes, though rarely, get into the blood stream. 

 I have never been able to observe one in the act of entering the 

 current. Yet in a quiescent state they might become sur- 

 rounded by endothelial cells along with the erythroblasts, and 

 finally be swept away. They might, on the other hand, actually 

 migrate through the porous wall of an early vessel. 



The enormous brown pigment cell presents a smooth circular 

 outline as it is carried along in the blood current. On account of 

 its size the chromatophore often meets with difficulties in passing 

 narrow portions of the vascular system. Several such cells 

 were seen in the blood circulation of different embryos during the 

 course of the observations, and when once located in the current 

 the same cell could be seen periodically for a long time as it came 

 around again and again through the vesssel within the field of 

 study. There is no question of the identity of these cells, as 

 their characteristic reddish brown color and coarse granular struc- 

 ture is readily recognized. It is most improbable to think of 

 them as becoming changed into any type of blood corpuscle, and 

 it is doubtless entirely by accident that they occasionally become 

 entrapped within the vessel wall and washed away by the current. 



c. Behavior of the chromatophores in specimens with no cir- 

 culation. The behavior of both the black and brown types of 

 pigmented cell is distinctly different in embryos without a circu- 

 lation of the blood from that described in the two previous sec- 

 tions for normal embryos. 



