varying much in size, according to the amount of food-yolk with which it is supplied. Like other cells, it has a nucleus, and this undergoes changes which are definitely related to the fertilization of the ovum, which we describe as the biological end. Such preparatory changes for a future contingency are especially characteristic of organic behaviour. There is nothing like it in the mineral kingdom. The nucleus divides into two parts, one of which passes out of the ovum and is lost. The nucleus again divides, and again one part passes out and is lost. Thus only one quarter of the original amount of nuclear matter remains. Now, division of the nucleus occurs whenever an animal cell divides; but in this case (apart from details which would here be out of place) there is this difference. During the ordinary division of cells there are found in the nucleus a definite number of curved rods, and this number is constant for any given species; but in the nucleus which remains in the ovum after three parts of its substance are lost, the number of rods has been reduced to half that which is common to the species. The egg is now ready for fertilization. A minute active cell, which is produced by the male, and which also has only half the normal number of rods, enters the ovum. The two nuclei approach each other, and give rise to the single nucleus of the fertilized ovum, which thus has the full number of rods—half of them derived from one parent, half from the other parent. The sperm cell of the male adds little to the store of protoplasm in the ovum; but it introduces a minute body, which seems to initiate subsequent divisions of the cell. The nature of these divisions may be seen in the accompanying diagrammatic figure. In A the cell is just preparing to divide. Above the nucleus is the minute body (centrosome) just spoken of, which has already divided. In the nucleus the matter of which the rods will be constituted is net-like. In B this net-work has taken on the new form of a coiled thread, while the divided body above is associated with a spindle of delicate fibres. In C the membrane round the nucleus has disappeared, and the coiled thread has broken up into curved rods (chromosomes), four of which are shown. The two halves of the minute body form the centres of radiating stars. In D each curved rod has split along its length, and the two parts are being drawn asunder towards the centres of the two stars; the cell itself is beginning to divide. In E the process is carried a step further, while in F the cell has completely divided into two: the rods have disappeared as such, and are replaced by a net-work; a new nuclear membrane has been formed, and the minute body has again divided preparatory to the further division of the cell.
Fig. 3.—Cell-division.
Such, stripped as far as possible of technicalities, are some of the facts concerning the behaviour of cells and their nuclei during the process of cell-multiplication. No good purpose would be subserved by pretending that we fully understand them. The splitting of the rods does indeed seem an efficient means to the end of securing a fair division of the nuclear substance, which, according to many biologists, is the organic bearer of hereditary qualities in the cells. But that is nearly all that we can say. Is the process accompanied by some form of sentience? We do not know. That it is controlled and guided by any consciousness in the cell is most improbable. But if it be a purely organic and unconscious process it should at least impress on our minds the fact that such organic behaviour may reach a high degree of delicacy and complexity.
III. Corporate Behaviour
The word “corporate” is here applied to the organic behaviour of cells when they are not independent and free, but are incorporated in the animal body, and act in relation to each other. If the behaviour of the individual cell during division impresses us with the subtle intricacy of organic processes, the behaviour of the growing cell-republic during the early stages of organic development must impress us no less forcibly. We place the fertilized egg of a hen in an incubator, and supply the requisite conditions of warmth, moisture, and fresh air. Before the egg is laid cell-division has begun. A small patch of closely similar cells has formed on the surface of the yolk. Further subdivision is then arrested until the warmth of incubation quickens again the patch into life. But when once thus quickened no subsequent temporary arrest is possible—life will not again lie dormant. If arrest there be it is that of death. And from that little patch of cells, which spreads further and further over the yolk, a chick is developed. Into the intricate technicalities of embryology this is not the place to enter. But it is a matter of common knowledge that, whereas we have to-day an egg such as we eat for breakfast, three weeks hence we shall have a bright active bird, a cunningly wrought piece of mechanism, and, more than that, a going machine. During this wonderful process the cellular constituents take on new forms and perform new functions, all in relationship to each other, all as part of one organic whole. Here bones are developed to form a skeletal framework, there muscles are constituted which shall render orderly movements possible; feathers, beak, and claws take shape as products of the skin; gut and glands prepare for future modes of nutrition; heart and blood-vessels undergo many changes, some reminiscent of bygone and ancestral gill-respiration, some in relation to the provisional respiration of the embryo by means of a temporary organ that spreads out beneath the shell, some preparatory to the future use of the lungs—some, again, related to the absorption of food from the yolk, others to subsequent means of digestion; nerve, brain, and sense-organs differentiate. A going machine in the egg, the chick is hatched, and forthwith enters on a wider field of behaviour. Few would think of attributing to the consciousness of the embryo chick any guiding influence on the development of its bodily structure, any control over the subtle changes and dispositions of its constituent cells. But no sooner does the chick, when it is hatched, begin to show wider modes of instinctive behaviour, than we invoke conscious intelligence for their explanation, seemingly forgetful of the fact that there is no logical ground for affirming that, while the marvellous delicacies of structure are of unconscious organic origin, the early modes of instinctive behaviour are due to the guidance of consciousness. Such modes of behaviour will, however, be considered in another chapter. Here we have to notice that the unquestionably organic behaviour of the incorporated republic of cells may attain to a high degree of complexity, and may serve a distinctly biological end.
Fig. 4.—Wapiti with antlers in velvet.
There is, perhaps, no more striking instance of rapid and vigorous growth than is afforded by the antlers of deer,[3] which are shed and renewed every year. In the early summer, when growing, they are covered over with a dark hairy skin, and are said to be “in velvet.” If you lay your hand on the growing antler, you will feel that it is hot with the nutrient blood that is coursing beneath it. It is, too, exceedingly sensitive and tender. An army of tens of thousands of busy living cells is at work beneath that velvet surface, building the bony antlers, preparing for the battles of autumn. Each minute cell, working for the general good, takes up from the nutrient blood the special materials it requires; elaborates the crude bone-stuff, at first soft as wax, but ere long to become hard as stone; and then, having done its work, having added its special morsel to the fabric of the antler, remains embedded and immured, buried beneath the bone-products of its successors or descendants. No hive of bees is busier or more replete with active life than the antler of a stag as it grows beneath the soft, warm velvet. And thus are built up in the course of a few weeks those splendid “beams,” with their “tynes” and “snags,” which, in the case of the wapiti, even in the confinement of our Zoological Gardens, may reach a weight of thirty-two pounds, and which, in the freedom of the Rocky Mountains, may reach such a size that a man may walk, without stooping, beneath the archway made by setting up upon their points the shed antlers. When the antler has reached its full size, a circular ridge makes its appearance at a short distance from the base. This is the “burr,” which divides the antler into a short “pedicel” next the skull, and the “beam” with its branches above. The circulation in the blood-vessels of the beam now begins to languish, and the velvet dies and peels off, leaving the hard, bony substance exposed. Then is the time for fighting, when the stags challenge each other to single combat, while the hinds stand timidly by. But when the period of battle is over, and the wars and loves of the year are past,