The effect of genic time‐factors upon growth and differentiation as observed in certain snail shells, bivalves and plants

Structural genes do not all function at the same time but are controlled by time‐factors acting as operators. (1). Certain genes, involved in the production of shells in snails and bivalves, function only during the initial stages; these genes then become inactive and the formation of the shell is continued by other genes. A phase‐change of this type is found in the genus Voluta. (2). Such alternating periods of activity and inactivity can be confined to the same set of structural genes. This will result in shells composed of identical sectors but increasing in size. This sectorial growth is observed in numerous genera and is particularly conspicuous in the genus Murex. Time‐factors controlling both phase‐change and sectional growth can be regarded as general. (3). Time‐factors determining various types of shell decoration are specific in nature. The time‐factors are local , i.e. limited to the site at which they act as seen in a specimen of Phalium strigatum. They also appear to act individually as observed in Mitra mitra and Conus tessulatus. In the bivalve Tellina evidence was found to indicate that the activating factor multiplies with the cell and in this respect behaves as a gene . (4). The time‐factors can be correlated in different ways, negatively as well as positively. Various patterns resulting from such correlations are discussed. Such mutual correlations are of great importance for an understanding of the ontogenetic development.