DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 123 



It must be appreciated, however, that some of the difference in 

 extent is due to the flattening of the cells in figure 14. 



Figure 14 shows two huge pigment cells on the yolk-sac of a 

 5 day embryo in the act of arranging themselves along a vessel 

 wall. The granules are not so densely arranged as in the younger 

 stages, since the cell body is greatly thinned out in pressing 

 around the vessel. A number of granules are often arranged in 

 solid black lines and masses as indicated in the figure. 



The two cells are close together and a very peculiar phe- 

 nomenon is taking place. Each cell sends out short processes to 

 meet similar processes from its neighbor. The processes fuse, 

 and finally the two cell bodies melt into one thus forming a pig- 

 mented syncytium about the vessels of the yolk-sac. The 

 syncytia continue to expand along the vessels as enclosing sheaths 

 (fig. 15) . The dense black of the young chromatophores becomes 

 a steel grey as the granules are more thinly spread along the 

 vessels. 



In order to test whether the cells had actually joined or fused 

 to form a true syncytium, I attempted to contract them, think- 

 ing that this should pull them apart unless they were actually 

 united. The various solutions of KCl which Dr. Spaeth has 

 found to contract the chromatophores within the embryo's body 

 failed entirely to produce any change in the chromatophores of 

 the yolk-sac. Solutions of adrenalin of one to 1000, one to 10,000 

 and one to 100,000, which Dr. Spaeth so kindly supplied me, 

 were then tried. These solutions contract the pigment cells on 

 the brain of the embryo until they appear as small black dots, 

 but neither the black nor brown chromatophores on the yolk-sac 

 respond in the slightest degree. Such specimens were preserved 

 to show the extreme contraction of the chromatophores over the 

 brain of the embryo in contrast to the unchanged pigment cells 

 of the yolk-sac. 



Fig. 11 A black and brown chromatophore lying in contact on a j'olk-sac 

 of 72 hours. The black cell is much the larger with broader pseudopod-like proc- 

 esses; both are in active movement as shown by comparing figure 12, of the same 

 two cells 15 minutes later and figure 13, the same cells 20 minutes after figure 

 12. (3b. DD. ob.). 



