6 JACQUES LOEB 



the embryos are very obvious. The embryos are rather small, 

 owing to the slowness with which they digest the yolk. Their 

 eyes are abnormal and approach the cyclopean condition. In 

 many specimens only irregular masses of pigment indicate where 

 the eyes should be. The head is comparatively small and not 

 bent as is characteristic for the pure breed. The heart is devel- 

 oped but corresponds to an early stage in the development. It 

 beats regularly and at an almost normal rate. The main blood 

 vessels exist and haemoglobin is formed but the creeping of the 

 pigment cells upon the blood vessels does not take place. 



Years ago I found that the marking of the yolk sac of Fundulus 

 and of the embryo is caused by the creeping of the chromatophores 

 upon the blood vessels. I showed that this phenomenon is 

 due to a tropism which depends upon the circulation. When the 

 circulation was suppressed pigment was developed but the chro- 

 matophores did not creep upon the blood vessels. At that time 

 I had succeeded in suppressing the circulation for a few days.^ 

 In the new experiments the hybrid embryos lived for a month 

 or more with pigment but without a circulation. They demon- 

 strate the correctness of my former statement, inasmuch as the 

 creeping of the chromatophores upon the blood vessels did not 

 take place. They also confirm the statement, that the forma- 

 tion of pigment cells is independent of the circulation. Newman 

 seems to hold the opposite view, but he evidently did not test 

 his assertion experimentally. 



These hybrids were also smaller than the pure breeds of the 

 same age, owing to the fact that the yolk is less rapidly digested 

 in the hybrids than in the pure breeds. This is a very important 

 link in our conclusions on heredity. The development of heredi- 

 tary characters is the result of the nature and the velocity of chem- 

 ical reactions between the mass of yolk on the one hand and the 

 substances in the nucleus, especially the chromosomes, on the 

 other. If two closely related forms be crossed, the chemical 

 reactions are not materially different in quality and velocity from 

 those of the pure breeds. But when distant forms are crossed it 

 is to be expected that greater differences in the nature and the 



' Loeb, Jour, of Morph., vol. 8, 1893; Woods Hole Lectures. 



