Influence of Environment on Animals 223 



The heliotropism of animals is identical with the heli- 

 otropism 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 foimd 

 in plants. 



We may briefly discuss the question of the transmission 

 through the sex-cells of such instincts as are based upon heli- 

 otropism. 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 of a pho- 

 tosensitive substance in the eyes (or in the skin) of the animal. 

 For the transmission of this the gametes need not contain 

 anything more than a catalyzer or ferment for the synthesis 

 of the photosensitive 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 (Fig. 36, p. 106). There is at that time no definite 



