298 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



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 formed but the chromatophores did not creep upon the 

 blood vessels. At that time I had succeeded in suppressing 

 the circulation for a few days. 1 In the new experiments the 

 hybrid embryos lived for a month or more with pigment, but 

 without a circulation. They demonstrate the correctness of 

 my former statement, inasmuch as the creeping of the chro- 

 matophores upon the blood vessels did not take place. They 

 also confirm the statement that the formation 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 are 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 impor- 

 tant link in our conclusions on heredity. The development of 

 hereditary characters is the result of the nature and the velocity 

 of chemical reactions between the mass of yolk on the one hand 

 and the substances in the nucleus, especially the chromo- 

 somes, on the other. If two closely related forms be crossed, 

 the chemical reactions need not be 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 rate of chemical reactions will be found 

 and the outcome will be pathological embryos and very likely 



1 Loeb, Jour. Morphol., VIII, 161, 1893; Mechanistic Conception of Life, p. 105 

 1912. 



