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Figure 3–5. Fertilization and division of cells in parasitic worms Ascaris (After van Beneden, 1883; From A. Mirsky. In: Molecules and Cells, vol. 4, 1969). 1 — approach of the nuclei of spermatozoon and ovum; 2, 3, 4 — the nuclear envelope is break down and the chromosomes are clearly discernable; in every nucleus there are two chromosomes — a half of the normal diploid chromosomal set; 5, 6 — preliminarily, each chromosome is divided in two; 7 — the divided chromosomes separate; 8, 9, 10, 11 — two centrioles appear, a dividsion spindle is formed between them; cell division begins; 12 — two cells are formed, there are four chromosomes in each nuclei. Two of them originate from the ovum, the other two — from the spermatozoon.
The nucleus or the Nuclear Equivalent (Nucleid) – Obligatory and Irreplaceable Organelle of Each Cell
Section 3.3. The discovery of cell nucleus is an important stage in understanding its
biological essence and development. According to literary data (see
Katznelson, 1939, 1963; History of Biology, 1972, 1975) the first who has
observed a nucleus in fish erythrocytes is Antony van Leeuwenhoek, but
his observations have not come to light. Fontana (1781) has depicted
nuclei in epithelial cells of snake epidermis. Then nuclei are observed by F.
Cavolini, J. Purkinje, Ch. Mirbel, F. Meyen, J. Müller, etc., but they have not
estimated their role and importance. At that time even the term “cell” have
not possessed a definite interpretation. Different terms have been used —
“grains”, “balls”, “pouchs”, etc. Recognizing the nucleus as a constituent of
plant cells is a merit of the English botanist Robert Brown (1833), who has