DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 541 



These large cells may be followed through their development 

 and they will be found to differentiate into one or the other of 

 the two types of chromatophores. The amoeboid cell shown in 

 different stages of movement already referred to as figure 4, 

 from a 52 hour embryo, is of this large type, and concluding 

 from my observations on great numbers of embryos, this is an 

 early condition of the future black chromatophore before any 

 pigment granules are deposited. 



Slightly older stages, figures 22 and 26, show the same cells 

 containing a light amount of pigment granules. Between the 

 second and third days the pigment granules appear and in an 

 embryo 72 hours old, end of the third day, the chromatophores 

 are already well differentiated freely moving huge cells. 



A black chromatophore from an embryo 72 hours old is shown 

 in figure 11 with one of its processes overlying the body of a 

 brown chromatophore, the type to be considered in the following 

 section. The black cell is loaded with coarse granules. The 

 nucleus occupies a central position and is clearly shown on ac- 

 count of the displacement of the pigment granules by its trans- 

 parent body. Several pseud opod-like processes project from 

 the chromatophore which is actively moving. The clear cyto- 

 plasmic tip of the pseud opod extends beyond the granular mass. 



Figures 12 and 13 are two other illustrations of the same cell 

 after 15 minute intervals. Its shape is constantly changing and 

 it is slowly moving in a direction towards the right side of the 

 page. The brown pigment cell is also moving and their rates of 

 progress are indicated by the increasing distance between them. 



This movement of the chromatophores continues until about 

 the end of the fourth or middle of the fifth day in the normal 

 embryos. By this time all of the black pigment cells of the 

 yolk-sac with few exceptions have taken up more or less perma- 

 nent positions along the walls of the blood vessels or around the 

 surface of the pericardial space. The individual chromatophores 

 have increased enormously in size as is seen by comparing figures 

 11, 12 and 13 with figure 14, all drawn at the same magnification, 

 though figure 14 is one-third more reduced in reproduction. 



THE AMERICAN JOURNAL OF ANATOMy , VOL. 18, NO. 3 



