GROWTH. — This is the biological problem of the increase in the volume of organisms during development. In mammals and human beings, it takes place in characteristic phases, the study of which, in the human species, is of great importance, since it is related to the constitutional type.
With the activation of the ovum, there is a first phase in which development without growth is observed, since the egg cell divides into progressively smaller cells (blastomeres) (segmentation phase) up to the morula stage. The increase in volume of the ger-
Cellular reproduction does not maintain a uniform rhythm in all the tissues of the embryo: fundamental in this regard are Bizzozero’s studies and his classification of tissues into tissues with perennial elements (which cease reproducing early, such as nervous tissue, and no longer possess the ability to reproduce), tissues with stable elements (which cease their reproductive activity with the end of bodily growth, such as the liver and other glands), and tissues with labile elements (which continue reproducing even after the phase of bodily growth throughout life, in order to compensate for their continuous functional wear, such as the epidermis and blood).
Growth is studied by relating weight or volume to age. Every organism has its own growth rhythm, and in general clearly defined periods of slowing and acceleration are observed in relation to the activity of particular endocrine glands (pituitary, thyroid, thymus, gonads). According to Quetelet, in man there is a maximum of growth from the sixth to the eighth month of intrauterine life, and postnatal maxima around the fifth and a half year and around the fifteenth–sixteenth year, the time of puberty.
If the growth curve of the most varied organisms, or of organs or parts thereof, is plotted, it can be seen that it has the form of an S. This curve has been compared with those of autocatalytic monomolecular reactions (Robertson, Ostwald). D’Ancona emphasizes the similarity of the curves of all those phenomena that limit themselves because of the state they themselves produce; just as in autocatalytic reactions there is self-limitation through the production of the substance itself, in organisms limitation is brought about by the differentiation of the tissues.
In the growth of organisms, every phase of differentiation is characterized by a slowing: sexual maturity produces the most considerable slowing or arrest of growth. In some cases, as in lampreys (Cotronei), there may even be a reduction in body size. In addition to hormonal and other internal factors, external and environmental factors act on growth: the quantity of food, temperature, the quantity of oxygen, and space are those most directly connected with II.