THE MARKING IN FISH EMBRYOS. 229 



tact with sea water, must give rise to polyps. Hence the 

 hereditary arrangement of organs is in such cases clue to the 

 fact that while one side of the germ comes in contact with 

 solid bodies, the other end remains in contact with sea water. 

 Let us next consider the hereditary power of assimilation. 

 We know that the corresponding proteids of a horse, a dog, 

 and a cat are different. If we feed a cat with horse flesh, it 

 transforms it into the specific proteids of a cat, while in a dog 

 the same horse flesh is transformed into the specific proteids of 

 a dog. For this transformation specific enzymes are necessary 

 which are different in both forms. As the fertilized ^g^ is 

 able to carry on processes of assimilation, we must assume that 

 such enzymes are transmitted through the &gg to the offspring. 

 The presence of enzymes or zymogens, from which the fer- 

 ments originate, is another circumstance in heredity which 

 none of the general theories has considered. We see thus that 

 what we call heredity is composed of very heterogeneous con- 

 stituents which cannot be explained by one single principle or 

 theory, unless the theory should become so indefinite as to 

 be almost meaningless. But such a theory would cease to be 

 of value. This paper will deal especially with the heredity of 

 the markings in fish embryos, which will offer another example 

 of the analytical study of heredity. 



II. 



Seven years ago I published a series of observations on the 

 origin of the markings in the yolk sac of the embryo of Fundu- 

 lus. The yolk sac of Fundulus possesses a tiger-like coloration. 

 Its origin is as follows : black and red chromatophores are 

 found on the surface of the yolk sac. They gradually creep 

 upon the blood vessels and form a sheath around them. This 

 creeping of the chromatophores upon the blood vessels is due 

 to a tropism of the former. I then believed it was chemotrop- 

 ism and assumed that the chromatophores were oriented by a 

 chemical constituent of the blood, especially by oxygen. But it 

 may be that it is stereotropism, or that both tropisms cooperate. 

 The heredity of the marking is, therefore, in this case deter- 



