Classics of Biology

By August Pi Suñer; Charles M. Stern | Go to book overview

CHAPTER VI
GROWTH AND REPRODUCTION

CELL DIVISION AND GROWTH

ALL cells have a certain limit for size, not varying appreciably from one to the other and measurable in thousandths or ten-thousandths of an inch. Of course some cells are larger than others but the differences in size are not really very great. On the other hand, however, we do find giant vegetables and animals living in the same environment as microscopic and practically invisible creatures and plant organisms, all of them from the least to the greatest containing a huge number of cells. The fact remains that the size of the cell element is practically constant, and that of the individual depends on the number of cells forming it, this being a characteristic index for any particular species.

Again, in reproduction through germ cells, the whole of the organism derives from progressive division of the originating cell, that is a zygote in the case of sexual reproduction or some other cell in place of the zygote when reproduction is asexual. From the division of such cell we get two blastomeres which split into four and these again into eight, then into sixteen, thirty-two, and so on to infinity. In this way the embryo keeps on increasing through multiplication of its cells, whilst after the embryonic stage growth still continues until the individual reaches adult size, its number of cells increasing all the time. Growth is therefore in all its aspects a consequence of cellular multiplication developing progressively until the volumetric limit for the species in question has been reached.

Thus we see why growth cannot go on indefinitely; it must cease as soon as the specific limiting size has been reached, and it is at this point generally that the reproductive faculties of the individual make their appearance. Reproduction becomes thus a continuation of the growth process. There is, indeed, an increase in mass in species because the number of individuals increases, but no single one of the species increases beyond its particular limiting volume, and when that is reached individual growth ceases. The division of its cells then serves only to replace those cells which have died off or, in the case of natural or artificial mutilation, to repair such damages.

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