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Since each of the four chromosomes thus splits, it follows that each of the two daughter nucleii will, of course, contain four chromosomes; two of which have been derived from the male and two from the female parent.
The number then in an egg which develops into a male is 2N-1, while other eggs undergo complete reduction and then have N chromosomes. The latter, however, do not develop until they have been fertilised. In the males, when mature, reduction takes place in the gametes, so that two kinds of sperms are formed, those with N chromosomes and those with N-l chromosomes.
We may assume that these units are represented in the hereditary substance of the cell-nucleus by definite bodies of too small a size to be seen, but constituting together the chromosomes.
The Mendelians used to say it was impossible to believe in the heredity of somatic modifications due to external conditions, because it was impossible to conceive of any means by which such modifications could affect the constitution of the chromosomes in the gametes within the modified body.
We have seen in the previous pages evidence for the wonderful nature of the chromosomes of the cells. We can not pretend to understand them, but they must be extraordinarily complex. We have seen proof that these chromosomes are probably the physical basis of heredity, since they are the only parts of each parent which are handed down to subsequent generations.
The original number of chromosomes in OEnothera is 14. In the mutation lata this has become 15, and also in another mutation called semilata. The chromosomes before the reduction division are arranged in pairs, each pair consisting, it is believed, of one paternal and one maternal chromosome.
When the germ-cells of the male and female make the division which marks the first step in reproduction, however, the process is different. Half the chromatin material passes into each of the two cells formed. This is called maturation, or the maturation division, and the new cells have only half the original number of chromosomes.
But this is still too many, for the egg is soon to unite with the male cell; and this male cell, as we shall see, is to bring in its own quota of chromosomes. Hence the egg must get rid of still more of its chromatin material. In the example figured, four is the normal number for the cells of the animal. The egg at the beginning of the process contained eight, but has now been reduced to two.
This process of the formation of the polar cells is thus simply a device for getting rid of some of the chromatin material in the egg cell, so that it may unite with a second cell without doubling the normal number of chromosomes. Completion of the process of extrusion of the chromatic material; fn shows the two chromosomes retained in the egg forming the female pronucleus.
Or it might be somehow due to what Morgan and his colleagues have called crossing over in the segregation of heterozygous chromosomes, so that a part corresponding to a sinistral body is united with a part corresponding to a dextral head. My conclusion from the evidence is that any process of congenital development may in particular zygotes exhibit a mutation, a departure from the normal.
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