THE MARKING IN FISH EMBRYOS. 



231 



Fig. 2. 



definite relation between the position of the pigment cells and 

 the blood vessels, and there is no definite marking. The same 

 is true in regard to the red pig- 

 ment cells. (The little spheres 

 are oil globules.) 



A few days later the number 

 of capillaries as well as that of 

 the pigment cells has increased 

 (Fig. 2), but a further change 

 is also noticeable ; each pigment 

 cell has crept upon a blood ves- 

 sel, and the shape of the black 

 pigment cells has changed some- 

 what. Now they are all rather 

 long ; that is, they follow the 

 direction of the blood vessels. 

 In the small ramified red pig- 

 ment cells no change of form is 

 as yet perceptible. Fig. 3 shows 

 a very characteristic condition, 

 such as is usually found toward 

 the end of the first week. The 

 black pigment cells follow the 

 blood vessels entirely. Where 

 a blood vessel branches off into 

 two parts the pigment cells show 

 the same ramification. The 

 black pigment cells now form a 

 sheath around the blood vessels. 



Fig. 4 shows the yolk sac 

 when the marking is almost com- 

 pleted. The black and red pig- 

 ment cells surround the blood 

 vessels so completely that no 

 one who sees the eggs for the 

 first time at this period could 

 realize that the black and red stripes in the yolk sac could 

 have originated from chromatophores. That part of the blood 



Fig. 3. 



Fig. 4. 



