240 
COLOUR IN NATURE 
CHAP. 
nective tissue cells of the body cavity, and migrates 
outwards through the blood-vessels in the wandering 
cells which fill up the cavities of these, or whether it 
may arise in part in the epidermis in situ , and con¬ 
fines himself to emphasising the fact of the relation 
between the pigment spots and the obliterated open¬ 
ings of the transverse vessels. When we remember, 
however, that the places where blood stagnates are 
especially liable to become deeply pigmented, it is diffi¬ 
cult to avoid the conclusion that the pigment directly 
originates from the degenerating blood which must 
accumulate at these points. The question has some 
bearing upon theories of the origin of colour and 
colour patterns, but is obscured by the usual diffi¬ 
culty of distinguishing between post and propter. 
In connection with this paper of Zenneck’s we 
may note an observation by Mr. J. Loeb on the rela¬ 
tion between the blood-vessels and the coloration of 
the yolk-sac in the embryos of the fish Fundulus. 
The yolk-sac has here a peculiarly tiger-like colour 
due to a combination of black and red chromato- 
phores. At the time of their first appearance the 
pigment cells are practically uniformly scattered, 
but as the blood-vessels develop and the blood 
begins to circulate, the chromatophores begin to 
migrate to the surface of the vessels, and ultimately 
pigment is only visible as a covering to the vessels. 
By an ingenious experiment with a heart poison 
(potassium chloride), Loeb convinced himself that 
the migration only occurred when the blood was 
actually circulating in the vessels of the embryo. 
He concludes that the coloration of the yolk-sac 
in Fundulus is due to a specific irritability of the 
