Embryonic and postnatal growth of rat and mouse. V. Prenatal growth of organs and tissues, general principles: allometric growth, absence of growth, and the genetic regulation of the growth process.

(1) The investigation yielded several general growth principles, i.e., types of growth occurring rather frequently: (a) allometric growth, (b) a non-changing growth rate, persisting when the growth rate changes in the majority of tissues, (c) a growth rate equal to that of the whole embryo, (d) absence of growth and (e) jumps, i.e., changes in the volume or surface of organs completed within about half a day or less and probably due to changes in the cell volume or surface area; these histological changes tend to start simultaneously with the changes in growth rate. (2) Growth constants tend to have a value of about 2.0 in the prenatal period, as in the post-natal period. During development, however, their value increases slightly but significantly. (3) Further arguments are given for the mitosis-regulating model described in Part III. (4) The mitosis-regulating model combined with the general growth principles derived here and in earlier parts of this report constitute a comprehensive model for the genetiv regulation of the growth process.