Part 4 (2/2)

The discovery that Mendel made with edible peas concerning heredity has been found to apply everywhere throughout the plant and animal kingdoms--to flowering plants, to insects, snails, crustacea, fishes, amphibians, birds, and mammals (including man).

There must be something that these widely separated groups of plants and animals have in common--some simple mechanism perhaps--to give such definite and orderly series of results. There is, in fact, a mechanism, possessed alike by animals and plants, that fulfills every requirement of Mendel's principles.

THE CELLULAR BASIS OF ORGANIC EVOLUTION AND HEREDITY

In order to appreciate the full force of the evidence, let me first pa.s.s rapidly in review a few familiar, historical facts, that preceded the discovery of the mechanism in question.

[Ill.u.s.tration: FIG. 45. Typical cell showing the cell wall, the protoplasm (with its contained materials); the nucleus with its contained chromatin and nuclear sap. (After Dahlgren.)]

Throughout the greater part of the last century, while students of evolution and of heredity were engaged in what I may call the more general, or, shall I say, the _grosser_ aspects of the subject, there existed another group of students who were engaged in working out the minute structure of the material basis of the living organism. They found that organs such as the brain, the heart, the liver, the lungs, the kidneys, etc., are not themselves the units of structure, but that all these organs can be reduced to a simpler unit that repeats itself a thousand-fold in every organ. We call this unit a cell (fig. 45).

The egg is a cell, and the spermatozoon is a cell. The act of fertilization is the union of two cells (fig. 47, upper figure). Simple as the process of fertilization appears to us today, its discovery swept aside a vast amount of mystical speculation concerning the role of the male and of the female in the act of procreation.

Within the cell a new microcosm was revealed. Every cell was found to contain a spherical body called the nucleus (fig. 46a). Within the nucleus is a network of fibres, a sap fills the interstices of the network. The network resolves itself into a definite number of threads at each division of the cell (fig. 46 b-e). These threads we call chromosomes. Each species of animals and plants possesses a characteristic number of these threads which have a definite size and sometimes a specific shape and even characteristic granules at different levels. Beyond this point our strongest microscopes fail to penetrate. Observation has reached, for the time being, its limit.

[Ill.u.s.tration: FIG. 46. A series of cells in process of cell division. The chromosomes are the black threads and rods. (After Dahlgren.)]

The story is taken up at this point by a new set of students who have worked in an entirely different field. Certain observations and experiments that we have not time to consider now, led a number of biologists to conclude that the chromosomes are the bearers of the hereditary units. If so, there should be many such units carried by _each_ chromosome, for the number of chromosomes is limited while the number of independently inherited characters is large. In Drosophila it has been demonstrated not only that there are exactly as many groups of characters that are inherited together as there are pairs of chromosomes, but even that it is possible to locate one of these groups in a particular chromosome and to state the _relative position_ there of the factors for the characters. If the validity of this evidence is accepted, the study of the cell leads us finally in a mechanical, but not in a chemical sense, to the ultimate units about which the whole process of the transmission of the hereditary factors centers.

But before plunging into this somewhat technical matter (that is difficult only because it is unfamiliar), certain facts which are familiar for the most part should be recalled, because on these turns the whole of the subsequent story.

[Ill.u.s.tration: FIG. 47. An egg, and the division of the egg--the so-called process of cleavage. (After Selenka.)]

The thousands of cells that make up the cell-state that we call an animal or plant come from the fertilized egg. An hour or two after fertilization the egg divides into two cells (fig. 47). Then each half divides again.

Each quarter next divides. The process continues until a large number of cells is formed and out of these organs mould themselves.

[Ill.u.s.tration: FIG. 48. Section of the egg of the beetle, Calligrapha, showing the pigment at one end where the germ cells will later develop as shown in the other two figures. (After Hegner.)]

At every division of the cell the chromosomes also divide. Half of these have come from the mother, half from the father. Every cell contains, therefore, the sum total of all the chromosomes, and if these are the bearers of the hereditary qualities, every cell in the body, whatever its function, has a common inheritance.

At an early stage in the development of the animal certain cells are set apart to form the organs of reproduction. In some animals these cells can be identified early in the cleavage (fig. 48).

The reproductive cells are at first like all the other cells in the body in that they contain a full complement of chromosomes, half paternal and half maternal in origin (fig. 49). They divide as do the other cells of the body for a long time (fig. 49, upper row). At each division each chromosome splits lengthwise and its halves migrate to opposite poles of the spindle (fig. 49 c).

But there comes a time when a new process appears in the germ cells (fig 49 e-h). It is essentially the same in the egg and in the sperm cells. The discovery of this process we owe to the laborious researches of many workers in many countries. The list of their names is long, and I shall not even attempt to repeat it. The chromosomes come together in pairs (fig. 49 a). Each maternal chromosome mates with a paternal chromosome of the same kind.

[Ill.u.s.tration: FIG. 49. In the upper row of the diagram a typical process of nuclear division, such as takes place in the early germ cells or in the body cells. In the lower row the separation of the chromosomes that have paired. This sort of separation takes place at one of the two reduction divisions.]

Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one of the divisions the double chromosomes separate so that each resulting cell comes to contain some maternal and some paternal chromosomes, i.e. one or the other member of each pair. At the other division each chromosome simply splits as in ordinary cell division.

[Ill.u.s.tration: FIG. 50. The two maturation divisions of the sperm cell.

Four sperms result, each with half (haploid) the full number (diploid) of chromosomes.]

The upshot of the process is that the ripe eggs (fig. 51) and the ripe spermatozoa (fig. 50) come to contain only half the total number of chromosomes.

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