Reductive Division (Meiosis) And Fertilization (Syngamy)

Very soon after the development of the individual has set in, a group of cells is reserved for producing at a later stage the so-called gametes, the sperma cells or egg cells, as the case may be, needed for the reproduction of the individual in maturity.

‘Reserved’ means that they do not serve other purposes in the meantime and suffer many fewer mitotic divisions. The exceptional or reductive division (called meio- sis) is the one by which eventually, on maturity, the gametes are produced from these reserved cells, as a rule only a short ‘The biologist will forgive me for disregarding in this brief summary the exceptional case of mosaics.

Meiosis ~(producing spores) Sporophyte ~ (diploid) ~ Fertilization Gametophyte ~ (haploid)

Fig. 5. Alternation of Generations.

time before syngamy is to take place. In meiosis the double chromosome set of the parent cell simply separates into two single sets, one of which goes to each of the two daughter cells, the gametes. In other words, the mitotic doubling of the number of chromosomes does not take place in meiosis, the number remains constant and thus every gamete receives only half - that is, only one complete copy of the code, not two, e.g. in man only 24, not 2 X 24 == 48.

Cells with only one chromosome set are called haploid (from Greek a1tAoG~, single). Thus the gametes are haploid, the ordinary body cells diploid (from Greek 817tAOG~, double). Individuals with three, four, … or generally speaking with many chromosome sets in all their body cells occur occasion- ally; the latter are then called triploid, tetraploid, … , polyploid.

In the act of syngamy the male gamete (spermatozoon) and the female gamete (egg), both haploid cells, coalesce to form the fertilized egg cell, which is thus diploid. One of its chromosome sets comes from the mother, one from the father.

Haploid Individuals

One other point needs rectification. Though not indispensable for our purpose it is of real in teres t, since it shows that actually a fairly complete code-script of the ‘pattern’ is contained in every single set of chromosomes.

There are instances of meiosis not being followed shortly after by fertilization, the haploid cell (the ‘gamete’) under- going meanwhile numerous mitotic cell divisions, which result in building up a complete haploid individual. This is the case in the male bee, the drone, which is produced parthenogen- etically, that is, from non-fertilized and therefore haploid eggs of the queen. The drone has no father! All its body cells are haploid. If you please, you may call it a grossly exaggerated spermatozoon; and actually, as everybody knows, to function as such happens to be its one and only task in life. However, that is perhaps a ludicrous point of view.

For the case is not quite unique. There are families of plants in which the haploid gamete which is produced by meiosis and is called a spore in such cases falls to the ground and, like a seed, develops into a true haploid plant comparable in size with the diploid. Fig. 5 is a rough sketch of a moss, well known in our forests. The leafy lower part is the haploid plant, called the gametophyte, because at its upper end it develops sex organs and gametes, which by mutual fertilization produce in the ordinary way the diploid plant, the bare stem with the capsule at the top.

This is called the sporophyte, because it produces, by meiosis, the spores in the capsule at the top. When the capsule opens, the spores fall to the ground and develop into a leafy stem, etc. The course of events is appropriately called alternation of generations. You may, if you choose, look upon the ordinary case, man and the animals, in the same way.

But the ‘gametophyte’ is then as a rule a very short-lived, unicellular generation, spermatozoon or egg cell as the case may be. Our body corresponds to the sporophyte. Our ‘spores’ are the reserved cells from which, by meiosis, the unicellular genera- tion springs.