The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

seem to have wills of their own, and dart actively and independently through the cytoplasm.

Most conspicuous among these bodies are certain regular globules, more or less numerous, which appear to the observer as brilliant shining spheres. These we now know are tiny droplets of oil. They move about the cell slowly and in groups; their wandering is purely passive, and they simply drift along, carried by currents in the cytoplasm. Scattered among these globules, less brilliantly luminous, and much smaller, is a host of fine granules. ‘These granules show irregular movements which, like those of the oil-droplets, are probably passive and due to currents in the transparent cell-fluid.

Very different from these granules and droplets are certain snake-like threads which writhe slowly through the cell. These threads are exceedingly fine and their length is variable; sometimes a thread can be seen to break into two, or two threads may join together end-to-end to form one. ‘Their movements are apparently spontaneous and independent of cytoplasmic currents. The threads are called mitochondria, and are composed of albumen and a fat-like compound, lecithin. ‘Though evidently of importance to the cell, their use has not yet been discovered. Lying like a cap over one end of the nucleus, a zone called the centroSphere may be distinguished. The principal function of this zone, which is dimly seen as a tangle of filaments and granules, will be described later on in connexion with celldivision. For the present its interest lies in the fact that it seems to be a region where mitochondria are made; they can be seen wriggling out from it into the cytoplasm.

Lastly, if the tissue under examination was taken from the eye of the embryo chick, there may be present among these other bodies within the cell a number of tiny rods of pigment. Sometimes these pigment-rods lie passively still, showing only a barely perceptible trembling movement. ‘This slight quivering is an example of what is known as Brownian movement ; it is due to the actual impact of molecules upon the sides of the rods. But from time to time these rods exhibit movements which it is hard to attribute to cytoplasmic currents; they dart to and fro ‘‘ more or less rapidly and irresponsibly, like guinea-pigs in a run.” We take our description from a paper by the late Dr. Strangeways and Dr. Canti.

Among this host of brilliantly illuminated bodies the outline of the nucleus is seen.

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THE SCIENCE OF LIFE

CHAPTER 2

The nucleus is a relatively large oval or spherical mass which drifts slowly to and fro in the cell. The boundary between nucleus and cytoplasm, like that between cytoplasm and culture-medium, has no visible wall. Inside the nucleus there are usually two mistily opaque bodies of irregular contour, the nucleoli; these bodies are continually changing their size, shape, and position. Except for the slowly writhing nucleoli nothing can be seen inside the nucleus but a clear fluid.

This, then, is a simple kind of cell, released from the body and leading a life of its own. The size of this object is such that about 2,500 laid side by side would measure an inch. And it is itself separately and independently alive. Such is the stuff that man and all his life is made of. In our bodies there are millions of such individual cells, inherent and necessary parts of us. They are not dead like the bricks in a wall; they are alive like the soldiers in an army. And they can be persuaded by the arts of Dr. Strangeways to desert! Then they will move by themselves, take nourishment, absorb oxygen, exude waste matters. They can be starved or suffocated. Not only will they move about as free individuals, but they will reproduce themselves. After a few hours some of the cells in these cultures will be seen to pull themselves into two paris, each of which lives and grows as an independent unit. They are no longer parts of an organized animal, yet so far from pining away in exile they are absorbing nourishing substances from the liquid about them, growing and ultimately performing an act of reproduction. In two or three days the multiplying and growing cells will have poisoned the serum with their own excretions, and the ‘“‘ culture” will die ; but this death can be prevented by putting a little of the failing culture into a drop of fresh serum, when the reproduction of the cells will resume and go on until that drop also is fouled. This administration of these exiled body cells is called sub-culturing. By subculturing every few days it is possible to keep

. strains of detached cells alive for months,

even years; indeed such strains may be kept going for periods of time far exceeding the individual life-span of the species from which they are taken. Cell-strains taken from a chick embryo have lived over fourteen years, while the ordinary life of a fowl is not a third of that span. No reason is apparent why we should not sub-culture for ever. If Dr. Strangeways had lived in the time of Julius Cesar and set a series of