ABSTRACT
The type of constitutional disturbance induced depends upon the developmental stage or moment during which the inhibiting agent or condition is effective. It is, therefore, clear that one and the same inhibiting agent, if acting at different developmental stages, may be experimentally employed to induce a great number of different developmental modifications. The structural response of the embryo is not specific for a given chemical or physical arresting agent, but it is specific for the embryonic stage during which the agent acts. There is in general a slight discrepancy in size and vigor between the bilateral halves of a vertebrate embryo. Not only does the struggle among the parts during development and growth occur between organs of different kinds, but very probably a similar competition takes place between the two members of a bilateral pair of organs or even among the parts within an organ.
