56 BIOLOGICAL LECrrRES. 



2. In adopting the theory of Sachs and applying it to 

 animal morphology, we must avoid a mistake very often made 

 even in the case of good theories, namely, the endeavor to 

 explain special cases which are complicated by unknown 

 conditions. Huyghens explained by his theory of light the 

 phenomena of refraction, but he could not and did not attempt 

 to explain the sensations of color. For these phenomena 

 the wave theory of light remains true, but color sensations 

 depend not only on the wave motion of the ether, but also 

 on the peculiar chemical and physical structure of the retina. 

 I think it perfectly safe to say that every animal has specific 

 eerm 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 and a falling back into the German Natur- 

 philosophie 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 haemoglobin, and so its forma- 

 tion 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. Later 

 on, however, all of the pigment cells have crept upon the 

 surface of the neighboring blood vessels. I succeeded experi- 

 mentally 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 chromatophore flow 



