But after a time, especially under unfavorable
circumstances, a new mode of reproduction appears. Certain cells
withdraw from the outer layer into the interior of the colony. Here
they are nourished by the other cells and develop into true
reproductive elements, eggs and spermatozoa. Fertilization, that is,
the union of egg and spermatozoon, or mainly of their nuclei, takes
place; and the fertilized egg develops into a new organism. But the
other cells, which have been all the time nourishing these, seem now
to lack nutriment, strength, or vitality to give rise to a new
colony. They die.
We find thus in volvox division of labor and corresponding
difference of structure or differentiation; certain cells retain the
power of fusing with other corresponding cells, and thus of
rejuvenescence and of giving rise to a new organism. And these
cells, forming a series through all generations, are evidently
immortal like the protozoa. Natural death cannot touch them. These
are the reproductive cells. The other cells nourish and transport
them and carry on the work of excretion and respiration. These
latter correspond practically to our whole body. We call them
somatic cells. In volvox they are entirely subservient to, and exist
for, the reproductive cells, and die when they have completed their
service of these.
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