Reproduction and Individual Development

There are two types of reproduction:

• Asexual: mitosis, budding, spores, vegetative.
• Sexual: fusion of gametes.

The formation of gametes is called gametogenesis. In males it is spermatogenesis, in females oogenesis. After fertilization, a diploid zygote is formed.

Individual development (ontogenesis) is divided into two periods:

• Embryonic: zygote, cleavage, blastula, gastrula, organogenesis.
• Postembryonic: from birth until death.

1. Zygote (2n)
2. Cleavage
3. Blastula
4. Gastrula (3 layers)
5. Organogenesis

Three germ layers form in the gastrula: ectoderm, endoderm and mesoderm.

Key facts

• Ectoderm: nervous system, epidermis of the skin.
• Endoderm: intestinal epithelium, liver, lungs.
• Mesoderm: muscle, skeleton, blood, kidneys.
• In spermatogenesis 4 spermatozoa form from 1 cell, whereas in oogenesis 1 complete egg cell is formed.
• Gametes are haploid (n), the zygote is diploid (2n).
• Complete metamorphosis: egg, larva, pupa, adult insect (butterfly, beetle). In incomplete metamorphosis there is no pupa (locust).
• In flowering plants there is double fertilization (S. G. Navashin): one sperm cell fertilizes the egg cell, the other fertilizes the central cell.