The Cambridge natural history, Vol. 01 (of 10)Hickson, Sydney J. (Sydney John)
Science
The Cambridge natural history, Vol. 01 (of 10)
Hickson, Sydney J. (Sydney John)
Animals
A temporary apocytial condition is often passed through in the formation of
the brood of cells by repeated divisions without any interval for
enlargement; for the nuclear divisions may go on more rapidly than those of
the cytoplasm, or be completed before any cell-division takes place (Figs.
31, 34, 35, pp. 95, 101, 104), the nuclear process being "accelerated" or
the cytoplastic being "retarded," whichever we prefer to say and to hold.
Thus as many as thirty-two nuclei may have been formed by repeated binary
subdivisions before any division of the cytoplasm takes place to resolve
the apocyte into true 1-nucleate cells.
In many cases of brood-formation the greater part of the food-supply of
the brood-mother-cell has been stored as reserve-products, which
accumulate in quantity in the cell; this is notably seen in the ovum or
egg of the Higher Animals. How great such an accumulation may be is
indeed well seen in the enormous yolk of a bird's egg, gorged as it were
to repletion. When a cell has entered on such course of "miserly"
conduct, it may lose all power of drawing on its own supplies, and
finally that of accumulating more, and passes into the state of "rest."
To resume activity there is needed either a change in the internal
conditions—demanding the lapse of time—or in the external conditions, or
in both.[46] We may call this resumption "germination."
Very often in the study of a large and complex organism we are able to
find processes in action on a large scale which, depending as they must
do on the protoplasmic activities of its individual cells, reveal the
nature of similar processes in simple unicellular beings: such a clue to
the utilisation of reserves by a cell which has gorged itself with them
so as to pass into a state of rest is to be found in that common
multicellular organism, the Potato. This stores up reserves in its
underground stems (tubers); if we plant these immediately on the
completion of their growth, they will not start at once, even under what
would outwardly seem to be most appropriate conditions. A certain lapse
of time is an essential factor for sprouting. It would appear that in the
Potato the starch can only be digested by a definite ferment, which does
not exist when it is dug, but which is only formed very slowly, and not
at all until a certain time has supervened; and that sprouting can only
{33}take place when soluble material has been provided in this way for
the growth of the young shoots. We have also reason to believe that these
ferments are only formed by the degradation of the protoplasm itself. Now
obviously this degradation must be very slow in a resting organism; and
any external stimulus that will tend to protoplasmic activity will
thereby tend to form at the same time the digestive ferments and dissolve
the stored supplies, to render them available for the life-growth and
reproduction of the being.
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