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. 
