THE CHROMOSOME MECHANISM 



shows US why the chromosomes must reproduce along a longitudinal 

 axis. Only by linear order and longitudinal doubling and splitting 

 can a chromosome, an aggregate of chromomeres, produce daughter 

 chromosomes, daughter aggregates, identical with each other and 

 with itself. The nucleus, therefore, evidently has the means of identical 

 self-propagation. 



What do we now know of the nucleus ? We saw that it is, in some 

 sense, paramount in the ceU; that is, in development and heredity. 

 We now see that it is an aggregate of particles, dissimilar particles, 

 and that this aggregate has the capacity of reproducing itself as an 

 aggregate of the same particles in constant proportions and constant 

 order. It undergoes great transformations in each cycle of nuclear 

 division, but they have a chemical regularity. Underlying this regu- 

 larity must be a constant molecular structure. It has a complex 

 character but it has the capacity for propagating this character 

 unaltered; in other words it has the capacity of heredity. 



The chromosome mechanism shown by mitosis makes it possible 

 to understand the constancy of vegetative growth and propagation. 

 It provides for that uniform genetic character of cells in a tissue 

 and in an organism on which, as we shall see later, their co-operation 

 in development is based. It shows why fragmentation of an organism, 

 from buds, cuttings and vegetative spores, gives daughter individuals 

 which are identical in heredity. It does not, however, explain the 

 differences between individuals, or the variation amongst the progeny 

 of the same individual or pair in sexual reproduction. To do this 

 we must examine the history of the nucleus during the sexual 

 processes. 



Meiosis 



We have seen that fertilization involves, or indeed consists in, 

 the fusion of two nuclei. The nucleus of the zygote produced by 

 fertilization thus contains the sum of the chromosomes of egg and 

 sperm. In the nematode Ascaris megalocephala chromosomes from 

 the two germ cells, one from each, combine to give two in the 

 embryo. In the flowering plants, Crepis capillaris or Crocus graveolens, 

 three chromosomes from the pollen nucleus combine with three 

 corresponding ones from the egg nucleus in the embryo sac to give 

 six in the embryo, three obvious pairs. In the formation of these 



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