Tropic reactions of tissue-cells 267 
The heliotropism of animals is identical with the heliotropism of 
plants. The writer has shown that the experiments on the effect of 
acids on the heliotropism of copepods can be repeated with the same 
result in Volvox. It is therefore erroneous to try to explain these 
heliotropic reactions of animals on the basis of peculiarities (e.g. 
vision) which are not found in plants. 
We may briefly discuss the question of the transmission through 
the sex cells of such instincts as are based upon heliotropism. This 
problem reduces itself simply to that of the method whereby the 
gametes transmit heliotropism to the larvae or to the adult. The writer 
has expressed the idea that 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 syn- 
thesis 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 stereo- 
tropism. 
(c) The tropic reactions of certain tissue-cells and the morpho- 
genetic 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 antici- 
pate that the tiger-like colouration of the yolk-sac in the later stages 
of 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 
