The Classical Physicist's Approach to the Subject

THE HEREDITARY CODE-SCRIPT (CHROMOSOMES)

I use ‘pattern’ as ’the four-dimensional pattern’.

meaning not only the structure and functioning of that organism in the adult, or in any other particular stage, but the whole of its ontogenetic development from the fertilized egg cell to the stage of maturity, when the organism begins to reproduce itself.

This whole 4D pattern is determined by the structure of that one cell, the fertilized egg.

Moreover, we know that it is essentially determined by the structure of only a small part of that cell, its nucleus. This nucleus, in the ordinary ‘resting state’ of the cell, usually appears as a network of chromatine, I distributed over the cell. But in the vitally important processes of cell division (mitosis and meiosis, see below) it is seen to consist of a set of particles, usually fibre-shaped or rod-like, called the chromosomes, which number 8 or 12 or, in man, 48.

But I should really to have written these illustrative numbers as 2 X 4, 2 X 6, … , 2 X 24, … , and I ought-to have spoken of two sets, in order to use the expression in the customary meaning of the biologist. For though the single chromosomes are sometimes clearly distinguished and individualized by shape and size, the two sets are almost entirely alike.

One set comes from the mother (egg cell), one from the father (fertilizing s perma tozoon). I t is these chromosomes, or probably only an axial skeleton fibre of what we actually see under the microscope as the chromosome, that contain in some kind of code-script the entire pattern of the individual’s future development and of its functioning in the mature state.

Every complete set of chromosomes contains the full code; so there are, as a rule, two copies of the la tter in the fertilized egg cell, which forms the earliest stage of the future individ ual.

In calling the structure of the chromosome fibres a code- script we mean that the all-penetrating mind, once conceived by Laplace, to which every causal connection lay immediately open, could tell from their structure whether the egg would develop, under suitable conditions, into a black cock or into a speckled hen, into a fly or a maize plant, a rhododendron, a beetle, a mouse or a woman. To which we may add, that the appearances of the egg cells are very often remarkably similar;

and even when they are not, as in the case of the compara- tively gigantic eggs of birds and reptiles, the difference is not so much in the relevant structures as in the nutritive material which in these cases is added for obvious reasons.

But the term code-script is, of course, too narrow. The chromosome structures are at the same time instrumental in bringing about the development they foreshadow. They are law-code and executive power - or, to use another simile, they are architect’s plan and builder’s craft - in one.

GROWTH OF THE BODY BY CELL DIVISION (MITOSIS)

How do the chromosomes behave in ontogenesis?

The growth of an organism is effected by consecutive cell divisions. Such a cell division is called mitosis. I t is, in the life of a cell, not such a very frequent event as one might expect, considering the enormous number of cells of which our body is composed. In the beginning the growth is rapid. The egg divides into two ‘daughter cells’ which, at the next step, will prod uce a genera tion of four, then of 8, 16, 32, 64, … , etc. The frequency of division will not remain exactly the same in all parts of the growing body, and that will break the regularity of these numbers. But from their rapid increase we infer by an easy computation that on the average as few as 50 or 60 successive divisions suffice to produce the number of cells2 in a grown man - or, say, ten times the number,2 taking into account the exchange of cells during lifetime. Thus, a body cell of mine is, on the average, only the 50th or 60th ‘descendant’ of the egg that was I.

IN MITOSIS EVERY CHROMOSOME IS DUPLICATED

How do the chromosomes behave on mitosis? They duplicate ‘Ontogenesis is the development of the individual, during its lifetime, as opposed to phylogenesis, the development of species within geological periods. 2Very roughly, a hundred or a thousand (English) billions.

• both sets, both copies of the code, duplicate. The process has been intensively studied under the microscope and is of paramount interest, but much too involved to describe here in detail. The salient point is that each of the two ‘daughter cells’ gets a dowry of two further complete sets of chromosomes exactly similar to those of the parent cell. So all the body cells are exactly alike as regards their chromosome treasure. I However little we understand the device we cannot but think that it must be in some way very relevant to the functioning of the organism, that every single cell, even a less important one, should be in possession ofa complete (double) copy of the code-script. Some time ago we were told in the newspapers that in his African campaign General Mont- gomery made a point of having every single soldier of his army meticulously informed of all his designs. If that is true (as it conceivably might be, considering the high intelligence and reliability of his troops) it provides an excellent analogy to our case, in which the corresponding fact certainly is literally true. The most surprising fact is the doubleness of the chromosome set, maintained throughout the mitotic divisions. That it is the outstanding feature of the genetic mechanism is most strik- ingly revealed by the one and only departure from the rule, which we have now to discuss.