300 STUDIES IN GENERAL PHYSIOLOGY 



make further experiments in this direction) also of the 

 embryo. The yolk-sac of the Fundulus embryo has a very 

 characteristic tiger-like marking in the second week. Numer- 

 ous chromatophores, which contain in part black, in part 

 reddish -brown, pigment, develop on the surface of the yolk- 

 sac of Fundulus. In the early stages of development, on 

 the third day, no definite relation can be discovered between 

 the circulatory system and the chromatophores. The 

 chromatophores are scattered about irregularly upon the 

 blood-vessels, and in the spaces between them. As soon as the 

 circulation is established, however, the chromatophores begin 

 to creep upon the vessels, and in the latter periods of the 

 development, from the tenth day on, the chromatophores 

 are no longer found in the spaces between the vessels, but 

 have all crept upon them. But that is not all. The chro- 

 matophores of the yolk-sac of Fundulus have the character- 

 istic amoeboid appearance as long as they lie in the spaces 

 between the vessels. Their diameter in any direction is 

 greater than the diameter of an average-sized blood-vessel, 

 and much larger than that of the capillaries. As soon as a 

 chromatophore has reached a blood-vessel, however, it 

 accommodates its entire mass to the surface of the blood- 

 vessel, so that it finally loses its amoeboid appearance and 

 apparently forms only a layer about the blood-vessel. The 

 chromatophore cannot leave the surface of the blood-vessel 

 after it has once reached it. This relation is most apparent 

 where a blood-vessel branches. The chromatophore then 

 branches in the same way as the blood-vessel. If the circu- 

 lation of the blood is prevented by the addition of KC1 to 

 the sea-water, the chromatophores and the blood-vessels 

 both develop, but the chromatophores do not creep upon 

 the blood-vessels. The tiger-like marking of the embryo- 

 sac of Fundulus is apparently, therefore, a function of the 

 circulation, in so far as the chromatophores are compelled to 



