The multiple unfolding that takes place is entirely dependent upon temporo-spatial overlapping, a continuous meeting in time and space, a sequence of events that must be precisely synchronized so that one phenomenon may act on another at the right time at the right place. The entire ontogenetic process must be seen as a precision schedule that determines the evolvement of a temporo-spatial pattern of interactions between cells and tissues.
Some geneticists believe that induction comes about through the biochemical alteration of regional, cellular environments (Bonner, 1952). These alterations have the effect of activating specific genes that had been present earlier but had been in some state of dormancy. Gene activation induces a new type of protein-synthesis within the cell, a molecular reconstitution, resulting in cell and tissue differentiation (Markert, 1963). Different internal environments activate different genes. Thus cells are acted upon by their environment which, however, is itself made up of cells and their metabolic products; a very complex chain of events ensues, until a relatively steady state, called maturity, is reached.
As cells become differentiated, various kinds of enzymes are produced by them that serve as catalysts for the biochemical reactions involved in development as well as in general metabolic function of the whole organism. The synthesis and biochemical structure of the enzymes are directly controlled by the molecular structure of genes, and small alterations in the latter (due to mutation) may easily affect the catalyzing efficiency of the enzymes and thereby change the temporal proportions of many far reaching reactions. The untoward temporal irregularities may affect growth rates by failing to initiate or inhibit growth activities, and this may result in irregularities of spatial contiguities and relations, thereby altering the entire spatial-temporal pattern. We see now how genes may be responsible for the inheritance of certain structural characteristics such as the famous Hapsburg lip, or a shortening of the chin, or excessively long legs. In these instances, growth is allowed to continue unhampered for a slightly longer time than is common, or it may be inhibited at a slightly earlier period.
But, as is well known, genes do not merely control the size and shape of structure but skills and capacities as well (Bernstein, 1925; Haecker and Ziehen, 1922, McClearn, 1964). These too may very well be due to spatio-temporal alterations in the ontogenetic schedules. For instance, the differential growth of internal or peripheral organs may clearly be accompanied by differences in capacities; enlarged heart and lung may improve the ability to run; an enlarged liver the endurance for prolonged intake of alcoholic beverages; a thinning of the fingers, the capacity for assembling electronic equipment. Some skills may beimproved through structural alterations that have the effect of lowering sensory thresholds, whereas the ability to dive may be enhanced by a heightening of tolerance for CO concentration in the blood.
More directly related to temporal events during ontogeny may be the prolongation of certain primitive undifferentiated stages. By postponing differentiation, either specific tissues or perhaps the entire developing individual, may become more susceptible to environmental influences (either the internal or external environment) and this may result in the creation of various types of critical periods such as have been briefly discussed in Chapter Four. These considerations make it clear that it is not strictly correct to speak of genes for long ears, for auditory acuity, or for the capacity for language. Genes can only affect ontogenesis through varying the cells’ repertoire of differentiation, but this, in turn, may have secondary effects upon structure, function, and capacities.
(2) Relative Growth
Certain aspects of growth can be quantified and treated mathematically. Let us merely refer to one instance, namely the phenomenon of allometric growth. Different portions of the body and limbs grow at different rates, and, therefore, an animal’s proportions are altered throughout development (Fig. 6.5). This is partly due to the existence of growth gradients (J. Huxley, 1932) along various axes of trunk and limbs. It has been found empirically (Reeve and Huxley, 1945) that the relation between the size and weight of two parts of the body (y and x) is that of an exponential function of the form
y = ax,
where a and b are constants. It is convenient to write this formula in its logarithmic form
log y = log a + b log x
and to plot measurements on double logarithmic paper so that all exponential functions appear as straight lines. If, for instance, we plot the weight of cats’ brains against the weight of the same animals’ body weights and take measurements at various stages of development, we find that the simple relationship, expressed by the allometric formula holds fairly well throughout ontogenetic development. The curves indicate differences in growth rate in various parts of the body and show that the proportion between such rates remains constant. The curves do not reflect the actual time it takes the animal to attain any of the values.