230 BIOLOGICAL LECTURES. 



mined by a stimulus which the blood vessels exercise upon 

 another tissue, namely, the chromatophores. Both tissues are 

 formed rather independently of each other, but from the fact 

 that the chromatophores must creep upon the blood vessels, 

 and that the latter have a hereditary arrangement, the marking 

 becomes hereditary too. This contradicts those theories of 

 heredity which try to derive all the peculiarities of the animal 

 from corresponding peculiarities of the sexual cell, for instance, 

 Weismann's theory. 



The observations I made on Fundulus have since been con- 

 firmed for other classes of animals too. Dr. Arnold Graf made 

 similar observations in leeches, and Zennek has found that the 

 longitudinal stripes in the Ringelnatter are determined by 

 blood vessels. But as he has overlooked my papers on the 

 subject he did not realize that the marking is produced by 

 the chromatophores being bound to creep upon the blood vessels. 

 I have continued my observations on the yolk sac and the em- 

 bryo of Fundulus, and will try to explain the history of the 

 origin of the marking in connection with some drawings. 



Fundulus has two kinds of pigment cells: (i) large black 

 cells with almost no ramifications and processes, and (2) small 

 red pigment cells with an enormous amount of ramification. 

 Both differ somewhat in their reactions. The black cells react 



and move quicker than the red 

 cells. In Fig. 3 the large black 

 pigment cells can easily be dis- 

 criminated from the star-shaped 

 red pigment cells. The latter 

 are characterized in the drawings 

 by fine lines. 



At first the yolk sac is free 

 from pigment cells. As soon as 

 the vessels and the blood corpus- 

 cles develop, the first pigment 

 cells make their appearance on 



the yolk sac. Fig. I shows this stage. Some of the large 

 black pigment cells are situated on blood vessels, others are 

 scattered in the gaps between the capillaries, but there is no 



