The document summarizes several historical theories of embryological development: 1) Preformation theory proposed that embryos existed preformed and fully formed within the egg or sperm, simply needing to grow larger. This was later disproven. 2) Epigenesis theory, advocated by Wolff, stated that differentiation occurs after fertilization from an undifferentiated egg or sperm, and is the universally accepted view. 3) Pangenesis theory proposed by Darwin involved "gemmules" transmitting traits, but lacked scientific basis. 4) Axial gradient theory proposed physiological gradients along the axis determine development and regeneration, with Child expanding on this view quantitatively.

1. Theories of preformation,
pangenesis, epigenesis, axial
gradient and germ plasm

2. Preformation theory
• During the 1700s in Europe, embryology
was the focus of a controversial and
lively debate that involved many of the
greatest and most celebrated scientists
and philosophers of that time. Most
scientists were convinced that all
embryos existed since Creation as
preformed miniatures, held within their
parents, ready to simply grow larger and
emerge when their time arrived.
Later, Jan Swammerdam (1637-1680)
stated a distinct theory of preformation
based on similar research with insect,
chicken, and frog eggs (1669).
Swammerdam's ideas on preformation
were further expanded in 1674 by
Nicolas de Malebranche, who
postulated emboïtement, or the
encasement theory, in which completely
preformed embryos were arranged in a
box within a box system.

3. • Preformationists believed that all living organisms,
plant and animal, were created in complete but
miniature form, within the eggs that each parent
contained. Emboïtement postulated that all future
generations existed in a long series of miniature,
complete embryos held in place and awaiting their
appointed time to grow and emerge.
• The development of zygote resulted only in the growth
of miniature human, who was already present in the
egg and sperm. However, this theory was soon given
up because this could not be proved scientifically.

4. Epigenesis:
• This theory was advocated by Wolff (1738—
1794), a German biologist. This theory states that
egg or sperm cells do not contain miniature
human. In other words, egg or sperm cells are
undifferentiated.
• The differentiation into various organs or parts
takes place only after fertilization from the zygote
resulting into development of adult tissues and
organs. This concept is known as epigenesis
which is universally accepted.

5. • William Harvey (1578-1657), studying chicken and
deer embryos, reached the same conclusion. Harvey
stated that all life comes from an egg and that the
embryo builds its organs and other parts individually, in
succession and due order, in a process he termed
"epigenesis." And like Aristotle, Harvey also postulated
that a life force, which he called the "generative
principle," initiated the embryo's growth after
fertilization. Later, Caspar Friedrich Wolff (1738-1794)
would take the lead in researching and advocating a
theory of epigenesis that would counter the
preformationists and their often implausible
explanations.

6. Pangenesis
• This theory was proposed by Charles Darwin (1809—
1882), an English naturalist. This theory states that very
small, exact but invisible copies of each body organ and
component (called gemmules) are transported by the
blood stream to the sex organs. These invisible copies
of each body organ are called the gemmules.
• These gemmules are assembled in the gametes. After
fertilization these gemules move out to different parts
of the body resulting in the development of respective
organ. A defective gemmule will lead to the
development of defective organ in an individual. This
theory was given up because it did not have scientific
basis.

7. Germ Plasm
• This theory was advocated by August Weismann
(1889), a German biologist. This theory states that
body tissues are of two types, viz., germplasm and
somatoplasm. The germplasm refers to the
reproductive tissues or cells which produce gametes.
• The somatoplasm includes all other body tissues which
are not related to sexual reproduction. Thus,
transmission of characters from one generation to
other takes place only through germplasm. Any change
in the germplasm will lead to change in the next
generation. This theory is accepted in a broad sense.

8. Axial Gradient Theory
• Original gradient theory was given by T. Boveri and was supported by M.
C. Child. Child called it as a metabolic axial gradient theory. According to
it, the egg has distinct animal and vegetal poles. The cytoplasm of the
animal pole divides rapidly but the rate of cleavage in the vegetal pole is
slow. It indicates that the rate of metabolism is more in the animal pole
than the vegetal pole which is called as a metabolic axial gradient.
Horstadius and Runnstrom called it as double gradient theory. They
explain egg development on the basis of animal pole gradient and
vegetal pole gradient.
• Gradients brought together Child’s interest in development and his
fascination with the origins of individuality and organization. The theory
grew from his studies of regeneration, which were largely based on work
he conducted with invertebrates, such as planaria, a flat worm, and the
hydroid Tubularia. Child observed that regeneration occurred in a graded
process along the axis of the organism, with the characteristics of each
physiological process seemingly determined by its location along the axis.
To explain these observations, Child posited the existence of physiological
factors working to guide the regenerative process. He was convinced that
these differences along the gradients could be explained quantitatively.

9. • Child’s gradient theory suggested that an externally
induced metabolic gradient served as the foundation for
physiological integration. He described gradients as
“integrating factors” between the organism and the
environment and suggested they were generated by the
action of external factors on the protoplasm (or cytoplasm).
External forces were seen as the cause of quantitative
differences in the organism, which then led to the
organized process of differentiation during development.
Although he recognized that other theories had proposed
quantitative differences in the organism, such as the
existence of different entities, or “ formative stuffs,” he
insisted that quantitative metabolic differences in the
organism, or gradients, accounted for development. For
example, he suggested that when particular substances
gathered in a certain area of the organism, they did so in
response to a gradient.