ch 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 mesenchyme 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.

Kammerer has investigated the conditions for the laying of eggs in two
forms of salamanders, e.g. Salamandra atra and S. maculosa. In both
forms the eggs are fertilised in the body and begin to develop in the
uterus. Since there is room only