A CONTRIBUTION TO THE PHYSIOLOGY OF 

 COLORATION IN ANIMALS. 



JACQUES LOEB, 



Assistant Professor of Physiology, University of Chicago. 



1. It is the purpose of the following pages to communicate 

 some observations concerning one of the physiological causes 

 of coloration in animals, the term coloration meaning the 

 relative distribution and shape of the colored parts on the 

 surface of an animal. The reader knows that we have to 

 discriminate between two kinds of coloration in animals, 

 (i) coloration determined merely by the structure of the 

 superficial tissues, and (2) coloration produced by specific 

 pigments. In the second case the pigment may or may not 

 be exclusively contained in specific pigment cells, the chro- 

 matophores. It is my intention in this paper to deal with 

 coloration produced exclusively by chromatophores. 



For the physiologist the question arises which circumstances 

 determine such an arrangement of pigment cells as to produce 

 a fixed a;nd typical design. There are two possibilities; the 

 pigment cells are formed in the same place where they are 

 found, or are produced irregularly all over the surface, and by 

 secondary causes are forced to migrate to certain places and to 

 gather there. 



While studying the physiology of development in fish, I was 

 struck by the tiger-like coloration of the yolk sac of Fundulus. 

 The observation of the development of this coloration showed 

 me that in this case the coloration is produced by a migration 

 of the chromatophores from the place where they first appear 

 to the blood vessels, and this migration seems to be the result 

 of an attraction which the thin-walled blood vessels, or what 

 is more probable their contents, exert upon the amoeba-like 

 chromatophores. 



2. When the eggs of Fundulus are fertilized artificially, at a 

 temperature of about 20°, as a rule in three days the circula- 

 tion at the surface of the yolk begins, and at the same time 



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