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 



