998 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.! 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 
iLoeb, Jour. Morphol., VIII, 161, 1893; Mechanistic Conception of Life, p. 105 
1912. 
