130 CHARLES R. STOCKARD 



expansion and contraction would serve as vaso dilator and con- 

 tractor. Yet when they are along the vessel wall I have failed 

 to see them contract or expand even when treated with sub- 

 stances such as KC1 and adrenalin that violently contract the 

 chromatophores within the embryonic body. These experi- 

 ments have not been carried sufficiently far since they were 

 directed towards another point, still they indicate at least that 

 the pigment sheath of the vessel wall does not respond as a deli- 

 cate vaso-motor apparatus. 



Fig. 18 A group of brown, indicated in outline only, and black chromato- 

 phores on the yolk-sac of a 16 day embryo in which the blood has never circulated. 

 There is no arrangement of the pigment cells on vessels and no real syncytium 

 of black chromatophores as compared with the conditions in the normal embryo. 



Wenckebach ('86) found in certain pelagic eggs containing a 

 number of oil drops which invariably floated up that pigment 

 cells often completely surrounded the oil globules, and as he 

 thought prevented these globules from focussing heat or light on 

 parts of the embryonic body. The oil drops in the demersal 

 Fundulus yolk do not particularly attract the pigment cells and 

 they are rarely found to lie against the oil globule. 



The function of the pigment in the yolk-sac of Fundulus, if 

 it has any function other than its own existence, is most difficult 

 to determine. The same is true of the abundant pigment in the 

 coelomic wall and other internal structures of many animals. 



