THE MARKING IN FISH EMBRYOS. 233 



that the oxygen of the blood may be one of the causes that 

 force the chromatophores to creep upon the blood vessels. I 

 have expressed a similar belief in my former papers, but the 

 oxygen in the arterial blood is possibly not the only reason 

 why the chromatophores creep upon the blood vessels, since 

 wherever a vein is isolated they creep upon the vein too. Thus 

 it seems that positive chemotropism for oxygen is one stimulus, 

 but possibly not the only one that causes the migration of the 

 chromatophores upon the blood vessels. 



III. 



The further examination of the marking of the Fundulus 

 embryo has shown that the blood vessels are not the only 

 factors that determine the marking. A second factor is the 

 central nervous system. The back of the embryo is colored 

 black by pigment cells which follow the brain and the spinal 

 column also. They follow the outline of these organs almost 

 as closely as they follow the blood vessels. The details of the 

 origin of this marking must be reserved for future investigation. 



Everything we have said thus far about the marking of the 

 embryo refers to chromatophores, but in addition to chromato- 

 phores two other elements aid in producing the markings in 

 fish embryos. In certain places a diffused yellowish pigment 

 is deposited. It is a kind of excretion. The chemical char- 

 acter of this pigment and its origin have yet to be determined. 

 This pigment appears at a rather early stage in the develop- 

 ment. The second element is the structural colors. They do 

 not play as much of a role in the early stages of the embryo. 

 I intend to continue my study of the development of the mark- 

 ing in these embryos, especially in regard to the influence of 

 hybridization. I have already begun experiments on the latter 

 topic, but they are not far enough advanced to be published. 



Finally, may I be pardoned for adding a few remarks con- 

 cerning the theories of Eimer. I am no believer in Weismann's 

 theories of heredity, but it seems to me that the so-called theo- 

 ries of Eimer on the origin of the longitudinal striation in 

 animals are nothing but a play on words. One of his general 



