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 amceboid 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 amceboid 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 KCl 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 
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