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F I G . 7. Differentiation of somites in an isolate of Triturus ectoderm treated with a
solution of a protein sample (UISp) separated from guinea-pig bone marrow. Protein
concentration: 50/Mg/ml. χ 90.
A strange fact may be mentioned concerning the data presented in
Table III. If we compare induction frequencies of the nylon series and
the nylon-less series of one sample at a concentration suboptimal for
induction in the nylon series, the nylon-less series gives a frequency
higher than that of the nylon series (compare series 3 with 4, 9 with 10,
13 with 14 in the Table), although the nylon-less series gives a lower
frequency of induction if the comparison is made at a concentration
optimal for induction in the nylon series (compare series 7 and 8). The
apparently paradoxical result may be interpreted by the situation that
in the nylon technique the amount of medium for one explant is small
(ca. 80 μ\), while in the nylon-less series it is larger (1-3 ml.). It is
probable that at suboptimal concentrations the total amount of protein
and not the curling of the ectoderm is limiting the induction frequency,
while the reverse is the case at optimal concentrations.
having the smaller sedimentation constant is left in the supernate
(UISp), while the minor component having the larger constant is
sedimented (UlPt). All these data imply that the major component is
mainly responsible for the mesoderm inducing effect of fraction AP, and
hence represents the bone marrow factor itself. In good conformity with
this, sample UISp shows mesoderm inducing activity stronger than that
of sample AP.