Developmental rate and the formation of embryonic structures

The eggs of different species develop at specifically characteristic rates. These rates vary for a given species within certain limits. Variation in the direction of increased rate is far more limited and more difficult to bring about experimentally than is the opposite variation towards a decreased or slower rate.

The slight acceleration of developmental rate that may be induced in early embryos does not seem to cause any marked deviation from the normal course of development, on the contrary such embryos are unusually well developed. It might be said that the ideal rate of development is probably somewhat faster than the so-called normal average usually followed. In other words, the normal conditions of development are not entirely the best possible conditions.

The limits of retardation in developmental rates are extremely wide. The rate may be slowed down to almost zero or development may actually be to all appearances stopped for long or short periods of time in many species without noticeably injuring the embryo. Such an interruption normally occurs during the development of certain eggs as those of birds and some mammals. In eggs having a continuous development, such as those of fish, the rate of development may be slowed to apparent stoppage at many stages and held in such a condition [or some time without injury to the embryo which results after the inhibiting influence has been removed. However, when the rate of development is retarded but not entirely stopped at certain critical periods and development is allowed to proceed at this diminished rate for some time, most serious structural anomalies are induced.

Double monsters of varying dcgrecs of doubleness may actually be produced by slowing the rate at a time when the primary embryonic bud should arise.