268 Influence of environment on animals 



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 such observations as those made on Fundulus 

 might simplify the problem of the hereditary transmission of certain 

 markings. 



Driesch has found that a tropism underlies the arrangement of 

 the skeleton in the pluteus larvae of the sea-urchin. The position of 

 this skeleton is predetermined by the arrangement of the mesen- 

 chyme cells, and Driesch has shown that these cells migrate actively 

 to the place of their destination, possibly led there under the 

 influence of certain chemical substances. When Driesch scattered 

 these cells mechanically before their migration, they nevertheless 

 reached their destination. 



In the developing eggs of insects the nuclei, together with some 

 cytoplasm, migrate to the periphery of the egg. Herbst pointed out 

 that this might be a case of chemotropism, caused by the oxygen 

 surrounding the egg. The writer has expressed the opinion that the 

 formation of the blastula may be caused generally by a tropic 

 reaction of the blastomeres, the latter being forced by an outside 

 influence to creep to the surface of the egg. 



These examples may suffice to indicate that the arrangement 

 of definite groups of cells and the morphological effects resulting 

 therefrom may be determined by forces lying outside the cells. Since 

 these forces are ubiquitous and constant it appears as if we were 

 dealing exclusively with the influence of a gamete ; while in reality 

 all that it is necessary for the gamete to transmit is a certain form 

 of irritability. 



(d) Factors which determine place and time for the deposition 

 of eggs. 



For the preservation of species the instinct of animals to lay 

 their eggs in places in which the young larvae find their food and 

 can develop is of paramount importance. A simple example of this 

 instinct is the fact that the common fly lays its eggs on putrid 

 material which serves as food for the young larvae. When a piece 

 of meat and of fat of the same animal are placed side by side, the 

 fly will deposit its eggs upon the meat on which the larvae can grow, 

 and not upon the fat, on which they would starve. Here we are 

 dealing with the effect of a volatile nitrogenous substance which 

 reflexly causes the peristaltic motions for the laying of the egg in 

 the female fly. 



