all that is necessary for this transmission is the presence in
the eyes (or in the skin) of the animal of a photo-sensitive substance.
For the transmission of this the gametes need not contain anything more
than a catalyser or ferment for the synthesis of the photo-sensitive
substance in the body of the animal. What has been said in regard to
animal heliotropism might, if space permitted, be extended, mutatis
mutandis, to geotropism and stereotropism.

(c) THE TROPIC REACTIONS OF CERTAIN TISSUE-CELLS AND THE MORPHOGENETIC
EFFECTS OF THESE REACTIONS.

Since plant-cells show heliotropic reactions identical with those
of animals, it is not surprising that certain tissue-cells also show
reactions which belong to the class of tropisms. These reactions of
tissue-cells are of special interest by reason of their bearing upon the
inheritance of morphological characters. An example of this is found in
the tiger-like marking of the yolk-sac of the embryo of Fundulus and in
the marking of the young fish itself. The writer found that the former
is entirely, and the latter at least in part, due to the creeping of the
chromatophores upon the blood-vessels. The chromatophores are at first
scattered irregularly over the yolk-sac and show their characteristic
ramifications. There is at that time no definite relation between
blood-vessels and chromatophores. As soon as a ramification of a
chromatophore comes in contact with a blood-vessel the whole mass of the
chromatophore creeps gradually on the blood-vessel and forms a complete
sheath around the vessel, until finally all the chromatophores form a
sheath around the vessels and no more pigment cells are found in the
meshes between the vessels. Nobody who has not actually watched the
process of the creeping of the chromatophores upon the blood-vessels
would anticipate that the tiger-like colouration of the yolk-sac in the
later stages of the development was brought about in this way. Similar
facts can be observed in regard to the first marking of the embryo
itself. The writer is inclined to believe that we are here dealing with
a case of chemotropism, and that the oxygen of the blood may be the
cause of the spreading of the chromatophores around the blood-vessels.
Certain observations seem to indicate the possibility that in the adult
the chromatophores have, in some forms at least, a more rigid structure
and are prevented from acting in the way indicated. It seems to the
writer that su