REPRODUCTION IN UNICELLULAR ORGANISMS
Reproduction by division—In Amœbæ—In Infusorians—Divisions following one another in immediate succession—Formation of germ-cells in the Metazoa—Contrast between germ-cells and body-cells—Potential immortality of unicellular organisms—Beginning of natural death—Budding and division in the Metazoa.
We wish to consider the reproduction of organisms with special reference to the problem of heredity, and it is most instructive to begin with the lowest forms of life—the unicellulars—because their structure, as far as we can see with the instruments at our command, is very simple, and, what is even more important, is relatively homogeneous.
Fig. 59. An Amœba: the process
of division. A, before the
beginning of the division. B,
the nucleus divided into two.
C, the two daughter-Amœbæ.
Magnified about 400 times.
Suppose that there are bacteria-like organisms of quite homogeneous structure, and that these multiply by simply dividing into two, each rod-like creature dividing transversely in the middle of its length, the two halves would represent independent daughter-organisms, whose structure would correspond exactly with that of the mother-organism, could not indeed in any way deviate from it, and consequently would take over all its characters, that is, would inherit them. The size of body is the only feature which is not obviously inherited, but in reality it is potentially heritable, since the structure of the divided portions involves the capacity and the limits of their possible growth. Moreover, the size of body is not invariable in any species; a particular size is only reproduced under similar conditions of development. Inheritance here consists simply in a continuation of the mother-organism into its two daughter-cells.
Even in an Amœba (Fig. 59) we might picture the process of inheritance as equally simple, though in so doing we should probably be making a fallacious inference, for the structure of these lowest unicellular animals probably seems to us simpler and more homogeneous than it really is. Among Infusorians it is quite obvious that inheritance implies more than the mere halving of the mother-animal into the two daughter-cells; something more must be involved. For among these unicellular animals the differentiation of the body is not only great, but it is unsymmetrical. The posterior and the anterior ends are different, and the transverse division of the animal, in which the process of reproduction here consists, does not produce two halves, but two very unequal portions. In the division of Stentor, the so-called trumpet-animalcule ([Fig. 60]), the anterior portion contains the funnel-shaped mouth and gullet with its complicated nutritive apparatus, the circular peristome with its spirally curved rows of composite ciliated plates, the so-called membranellæ, and so forth; the posterior half contains nothing of all this, but possesses the foot of the mother-Stentor with its attaching organ, which the anterior half lacks. But each of the two portions possesses the power of 'regeneration,' that is, it is able to develop anew the missing parts, mouth or foot, and so on. So that here there is no longer merely a simple continuance of the maternal organization in the daughter-animals, there is something new added, something which requires explanation; we are confronted with the first riddle of heredity. Simple growth does not explain the phenomenon, for what has to be added to complete the halved portions has a different structure, a different form, different accessory apparatus from any that the halves themselves possess. It in no way affects this state of matters that in the normal process of division in Infusorians the formation of the new mouth and peristome-region begins before the halves have actually separated, for even if a Stentor be cut in two artificially the cut halves form complete animals. And, indeed, a Stentor may be cut into three or four pieces, and in certain conditions each piece will develop into a complete animal. These pieces must therefore possess something more than the mere power of growth. We shall try later on to discover whether this marvellous invisible transmission of characters, this implication of the whole in each of the parts, can be in any way theoretically expressed and included in our scheme of conceptual formulation.
Fig. 60. Stentor rœselii, trumpet-animalcule. Process of division. wsp, ciliated spiral leading to the mouth (m); cv, contractile vacuole. A, in preparation for division, the nucleus (k) has coalesced into a long twisted band. B, a second ciliated spiral (wsp´) has begun to be formed; the nucleus (k) is contracted. C, just before the constricting off of the two daughter-Stentors. Magnified about 400 times. After Stein.
Now that we have become familiar with these facts it will no longer surprise us to learn that the reproduction of unicellular animals does not always depend on equal division, but that unequal spontaneous divisions are also possible, so that one or several smaller portions of the cell-body, containing a portion of the cell-nucleus, can separate off from the mother-animal. This occurs especially among the suctorial Infusorians or Acinetæ. In relation to the phenomena of inheritance the problem raised by the equal division of the Infusorians repeats itself, and it is in no way affected by the fact that equal division can take place several times, or many times in succession, so that from one animal a large number of small pieces of the same size may be produced in rapid succession. The characteristic marks of the mother-animal are not infrequently lost sight of, wholly or partially, when this occurs, and the divided portions seem to consist of a homogeneous cell-body and nucleus; but they possess the power of regenerating themselves, or of developing, if the expression be preferred, into animals similar to the maternal-organism. Such divided portions might very well be called germs, only it must not be forgotten that the relation of the mother-animal to these germs is a different one from that of a higher animal or plant to its germ-cells; the unicellular animal breaks up by continued division into these 'germs,' while the Metazoon continues its individual existence unimpaired by the production of its germ-cells.