Adaptability and Alterability of Structures 125 
development, that they can adapt their form to new 
conditions, and that their specific form is not determined 
by special determinants in the nucleus but by external 
stimuli.” 8° 
The stomach of the tern, which ordinarily feeds 
on fish, is lined by a soft mucous membrane. If 
one feeds it with wheat for a few weeks its stomach 
develops a superficial horny coat, its musculature is 
strengthened and it takes on the character of a gizzard.°° 
If these stomachs belonged to two varieties of the same 
species, Weismann would have no hesitation in attribut- 
ing the diversity to special, and thus different, determi- 
nants which, as the facts show, do not really exist. 
Loeb has demonstrated that the colored design of 
the yolk sac of a fish embryo (Fundulus) is not in itself 
predetermined, but depends upon the distribution of 
blood vessels. The pigment cells are at first distributed 
uniformly but when the circulation of the yolk sac is 
established, they migrate toward the vessels, attracted 
probably, as Loeb supposes, by a chemical substance in 
the blood, and give rise thus to a definite design. Graf 
has likewise recently demonstrated that the color designs 
of the leech are not themselves inherited, but that they 
depend upon the disposition of muscle fibers in which 
the amoeboid pigment cells lie. It would be absurd, 
concludes Wilson, to imagine in all of these cases a 
special series of determinants for each individual color 
design. 
"Oscar Hertwig: Zeit- und Streitfragen der Biologie. I. Pra- 
formation oder Epigenese? P. 48—a9. 
Delage: L’hérédité etc. P. 604. 
91E, B. Wilson: The embryological Criterion of Homology. Biol. 
Lect. at the Mar. Biol. Lab. of Wood’s Holl; Summer Session 1894. 
Boston, U. S. A., Ginn, 1896. P. 116. 
