FORM AND DYNAMICS OF REPRODUCTION
WHENEVER a cell divides either directly or by mitosis, the most striking thing is the sequence of events developing in the nuclear substance. The discovery of the various phases of karyokinesis was very soon followed by that of the morphology of fertilization or syngamy. Penetration of the nucleus by the male gamete or pronucleus is the natural way of exciting mitosis in the ovule through the ensuing action upon its nucleus, that is to say on the female pronucleus. The chromatic substance of both nuclei contributes on an equal footing to karyokinesis, from which division of the fertilized egg or zygote into two blastomeres comes about. This was observed by O. Hertwig as far back as 1875 and confirmed by van Beneden in 1883 and by Boveri in 1886 using material of a different nature and origin.
In 1865 Nägeli had assumed that germ-cells contained directional micelles. Kölliker ( 1875-85) held that the nucleus itself might be the carrier of hereditable properties, on the assumption that the "idioplasm" resided therein, and it remained only for the protoplasm to fulfil a nutritive function. Strasburger in 1876 describes the chromosomes, whilst van Beneden in 1883 and Rabl in 1885 indicate their significance. Weismann in 1883 stated that they are the bearers of characters transmissible through heredity. Of late the conviction has grown more and more pronounced that chromosomes play a predominant and often exclusive part in heredity. We may mention, for instance, the voluminous researches of Edmund Beecher Wilson, from 1903 onwards, with regard to chromosomes over an extremely varied range of species and their functions.
In fertilization, chromosomes from each of the two participating gametes draw up alongside each other and form half and half the branches of the mother aster or equatorial plate at the commencement of metaphase. When immediately the hitherto single aster starts to divide into two daughter-asters, it is obvious that these latter will likewise be made up, as to one half, of chromatin of paternal origin, whilst the other half will derive from the maternal source. Similarly,