Lec2 History Of Animal Development

Preformation

This theory states that a miniature being is present in the gametes and they just unfold and grow to become an offspring. Development is just a matter of growth. Backed by 18th-century science religion and philosophy.

Epigenesis

This theory states that an organism develops by differentiation from an unstructured cell (egg and sperm). Development depends on gene changes whose expression is influenced by the environment.

Nicolaas Hartsoeker (1656-1725)

He made a sketch of the homunculus a tiny preformed human whom he believed to exist in the head of spermatozoa. He supported preformation.

Aristotle (400BC)

The first known embryologist whose view supported epigenesis even before the term was coined. He believed that the embryo was formed from the menstrual blood (soil) as a result of that blood’s interaction with a male factor (seed) called the male dynamic (present in the semen). He supported embryogenesis.

Oviparity

Animals lay eggs outside their bodies.

Viviparity

Animals give birth to live young.

Ovoviviparity

Animals produce eggs with yolks but the eggs stay inside the mother's body.

Holoblastic Cleavage

Total or complete division of the egg.

Meroblastic

Partial division of the egg.

Marcello Malpighi (1672)

believed that an unincubated chick egg already has a great deal of structure. He observed the heart within 30 hours of incubation and noticed that it began to beat before the blood reddened. With this observation he believed that organisms were not constructed

Jan Swammerdam (1720-1793)

a known ovist who rejected theories about spontaneous generation and metamorphosis on religious grounds. He believed that living things developed in eggs and that growth was merely an increase in the parts already present rather than the development of new parts by chance. He supported preformation.

Charles Bonnet (1720-1793)

an ovist whose most important contribution to embryology was the discovery of parthenogenesis in aphids (1762) proving that asexual reproduction of offspring was possible. (McGeady et al 2013) He also postulated that each embryo probably contained even smaller embryos ad infinitum like an idealized Matryoshka doll. He supported preformation.

Nicolas Malebranche (1638-1715)

supported preformationism from a religious standpoint for he aligned this theory with the belief of divine creation. Like Charles Bonnet he believed in the concept of ad infinitum miniature beings that an infinite series of plants and animals was contained within the seed or the egg but only naturalists with sufficient skill and experience can detect their presence. He supported preformation.

William Harvey (1578-1657)

established the doctrine ex ovo omnia (all life originates from an egg). He suggested that an egg found inside the female became fertilized through a kind of infection set in motion by the sexual act. He supported epigenesis.

Caspar Friedrich Wolff (1734-1794)

He coined the term epigenesis. He described embryonic development in both plants and animals as a process involving layers of cells thereby refuting the accepted theory of preformation. He demonstrated that embryonic parts developed from tissues that have no adult counterparts like the heart blood and intestine.

Christian Pander (1774-1865)

He studied the chick embryo and identified primary germ layers found in triploblastic embryos. He stated that primary germ layers interact to form organs. He supported epigenesis.

Karl Ernst Von Baer (1792-1896)

The first to discover the notochord & the mammalian ovum. He made strong arguments against the“biogenetic” concepts of his contemporary Ernst Haeckel He supported epigenesis.

Von Baers Law

States that embryos of different species develop from general shared features to specialized species-specific traits. It proved that animals do not pass through the adult stages of lower animals during growth but instead gradually diverge from one another.

Comparative Embryology

study of similarities and differences in the embryo of different species. With their embryonic similarities are likely evidence of common ancestry.

Evolutionary Embryology

restructured comparative embryology with the TOE and gave it a new focus. It was observed that embryonic resemblances would be a very strong argument in favor of genetic connectedness of different animal groups.

Richard Owen

Gave the definition of homology and analogy.

Homology

same ancestry

Analogy

different ancestry

Tetratology

The study of substances potentially toxic to embryos and can cause birth defects.