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| Fertilization of the egg by the sperm |
In the previous post we saw that at the
beginning of the 19th century the theory of epigenesis seemed to have won the
game. However, after 1850, and for just over a century, a cascade of new
discoveries tipped again the balance towards the theory of preformation. Let us
see what they were:
- The existence of a
nucleus
within animal and plant cells.
- The confirmation that the nuclei of the male and female gametes are fused
during fertilization. This put an end to spermism
and ovism and made it clear that the new being begins in the
zygote.
- The confirmation that certain structures
(the chromosomes) appear during
cell reproduction inside the nucleus of the cell, which seem to play a
very important role. It was also found that in the chromosomal endowment of
the zygote, half of the chromosomes come from the father and the other
half from the mother.
- A new science (Genetics)
was originated in the experiments of Thomas Hunt Morgan, who showed that the chromosomes are linked to Mendelian inheritance.
- The works of Oswald Avery, who
showed that DNA, a nucleic acid that appears
in chromosomes, is the basis of
Mendelian inheritance.
- The discovery of the structure of DNA (a double helix), made in the early 1950’s
by Francis Crick, James Watson and Rosalind Franklin.
- The deciphering of
the genetic code, which took place during the 1950s and the 1960s.
