394 NOTES AND COMMENTS [DECEMBER 
and differences are due to real transmission of heritable qualities, and 
how far to similarity in the induced “modifications”; there is the 
analysis of the inheritance by statistical and experimental methods, the 
biggest result of which has been Galton’s law; and there is the at 
present almost unassailable problem of conceiving how the heritable 
qualities work their way into realisation during the process of develop- 
ment—a problem that leads us away from the strict problem of 
heredity to that of “the principles of development.” In short, we. 
hardly think that the serious student of heredity has ever thought that 
he was facing one problem to which it might be expected some day to 
find one answer. In any case, we cannot admire the ingenious 
observer’s phraseology when he says that “what we call heredity is 
composed of very heterogeneous constituents.” He speaks of the so- 
called theories of Eimer as “nothing but a play on words,” but might 
not Loeb strengthen his case by taking his own words more seriously ? 
The particular problem which Professor Loeb discusses is that of the 
tiger-like markings in the yolk sac of the embryo of the fish called 
Fundulus—a subject in regard to which he has previously published 
results. The origin of the coloration is as follows :—black and red 
chromatophores are found on the surface of the yolk sac; they gradually 
creep upon the blood-vessels and ensheath these, exhibiting chemo- 
tropism due especially to the oxygen of the blood, or stereotropism 
(another brave word), or both. “The heredity of the markings is, 
therefore, in this case determined by a stimulus which the blood- 
vessels exert 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 heredi- 
tary 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.” But this is 
lamentable confusion ; for no one surely has supposed that there are 
not analysable immediate conditions operative at every stage of 
development: the point of Weismann’s theory is that the inherited 
organisation determines the particular occurrence and sequence of these 
conditions, and is thus the primary though not the immediate cause of 
the results. 
Loeb gives an interesting figure of the tail of an embryo, in which 
the chromatophores are seen to have crept upon the median artery 
while the vein remains free. This suggests that the oxygen of the 
blood may be one of the causes that force the chromatophores to creep 
upon the blood-vessels. But this is not the whole reason, for wherever 
a vein is isolated they creep upon it too. Moreover, the back of the 
embryo is coloured black by pigment cells which follow the brain and 
the spinal cord. 
The observations are interesting, but they appear to us to have to 
