Physiological Morphology 105 



light remains true, but color sensations depend not only on the 

 wave motion of the ether, but also on the chemical and physical 

 structure of the retina. I think it perfectly safe to say that 

 every animal has specific germ substances, and that the germ 

 substances of different animals differ chemically. Its chemical 

 qualities determine that from a chick's egg only a chick can 

 arise. But it would be a mistake to attempt at present an 

 explanation of how the unknown chemical nature of the germ 

 determines all the different organs and characters that belong 

 to the species. For instance, the yolk sac of the Fundulus 

 embryo has a tiger-like coloration. We might say that these 

 markings may be due to a certain arrangement of molecules or 

 complexes of molecules (determinants), which later on give rise 

 to the colored places of the yolk sac, but I found that this 

 coloration originates in a manner much more simple. The 

 pigment cells are formed irregularly on the surface of the yolk. 

 The pigment is chemically closely related to hemoglobin, and 

 so its formation may from the first be connected with the 

 formation of the blood corpuscles. But the arrangement of 

 the pigment cells during the first days of development is not 

 such as to produce any definite markings. They lie upon the 

 walls of the blood-vessels as well as in the spaces between the 

 capillaries (Fig. 36). Later on, however, all of the pigment 

 cells have crept upon the surface of the neighboring blood- 

 vessels (Fig. 38). I succeeded experimentally in showing it to 

 be probable that some of the substances contained in the blood 

 determine this reaction. These substances, if they diffuse from 

 the blood-vessel and touch the chromatophore, make, according 

 to the laws of surface tension, the protoplasm of the chroma- 

 tophore flow toward and at last over the blood-vessel and form 

 a sheath around it, while the gaps between the blood-vessels 

 become empty of chromatophores. In this way the chroma- 

 tophores are arranged in stripes, and possibly changes in the 

 surface tension, and not a preformed arrangement of the germ. 



