Developmental biology studies how interacting processes generate an organism's shapes, sizes, and structures from embryo to adult. Early scientists like Hippocrates, Aristotle, and Galen made observations of embryonic development but believed embryos derived nourishment from the mother. In the 1500s, da Vinci conducted early dissections of human fetuses. In the 1600s, Harvey observed chick embryos and concluded the amniotic fluid was absorbed. Later, scientists like Wolff and Pander established epigenesis over preformation through experiments showing structures develop progressively. Spemann's organizer experiments in the 1920s demonstrated induction, wherein one group of cells causes changes in adjacent cells. This established embryology's genetic basis and developmental mechanisms. Cell differentiation,

2. DEVELOPMENTAL BIOLOGY
The science of explaining how a variety of interacting processes
generate an organism’s heterogeneous shapes, size, and
structural features that arise on the trajectory from embryo to
adult, or more generally throughout a life cycle (Love 2008b;
Minelli 2011a).

3. EGYPTIANS(1400 BC)
• Made reference to placenta as the seat of the external soul
• Did not consider embryo alive untill the baby was born
• Also had knowledge that eggs could be removed from nests and
artificially incubated in ovens.

4. HIPPOCRATES(460 BC- 370BC)
• Believed that the embryo began development
by extracting moisture and breath from the
mother .
• Human fetus gained nourishment by sucking
blood from the placenta.
• Described the stages of hatching of chicks
during the 20 days of their creation.

5. • Studied embryos of different organisms-
using bird’s eggs and dissecting
mammalian and cold-blooded embryos.
• Wrote ‘Treatise de generation animalium’
(Generation of Animals)
• Mentioned that male contributed semen
and female portion called catemania laid
the foundation of the embryo.
ARISTOTLE (384BC–322 BC)

6. GALEN(150AD -180 AD)
• Galen of Pergamos
• Wrote the book ‘On the Formation of the
Fetus’
• Believed that Umblical cord was necessary
for respiration
• Describes the fetal development and
structures that are now named Allantois,
Amnion and Placenta

7. LEONARDO DA VINCI
• In late 1400s and early 1500s
• Dissection of the human fetus and
quantitative measurement of
embryonic growth
• First to give evidence that embryos
change in weight, size and shape over
time

8. WILLIAM HARVEY(1578- 1667)
• used simple lenses to observe chick
embryos and made observation on
circulation of blood.
• Determined the position where the embryo
arises in an egg (white spot)
• Believed that Amniotic fluid was absorbed
into the blood of the embryo
and later, the fetus

9. MARCELLO MALPHIGI(1628-1694)
• Professor of medicine
• One of the first scientists to study structures
such as the lungs, kidneys, spleen, brain, and
skin
• First microscopic account of chick development
• Concluded that egg contained miniature chick.
• Responsible for rise of Preformationist doctrine.

10. PREFORMATION THEORY
The organism is preformed as a complete miniature structure
in the egg and simply grows larger as it develops.
OVISTS
Those who believed that preformed organisms existed in eggs
were called “Ovists”. This was supported by Jan
Swammerdam, Charles Bonnet, Albert von Haller etc.

11. THEORY OF HOMUNCULUS
The theory of homunculus suggested that a little human
embryo (homunculus ) was hidden in the head of every
sperm
SPERMISTS
The theory of Homunculus was supported by
“Homunculists” or “Spermists”. Anton van
Leeuwenhoek who observed human sperms through an
improved version of a microscope and Nicholas
Hartsoecker were some of the Spermists.

12. • those who believed that development proceeded progressively from
unorganized matter
• Epigeneticists asked the Preformationists to explain the following :
• Embryonic monsters(embryos with severe congenital malformations)
• Regeneration of star fish arm
• Series of arguments between Caspar Friedrich Wolff (Epigeneticist)
and Albert von Haller (Preformationist)
EPIGENETICISTS

13. CASPAR FRIEDRICH WOLFF
• In 1759 he demonstrated that chick
intestine forms by folding of tissue
that detaches from the embryo’s
ventral surface. The folds eventually
transform into a closed tube.
• De Formatione Intestinorum in
1768
THEORY OF EPIGENESIS
New structures arise by progressing
through a number of different stages.

14. CHRISTIAN PANDER (1794-1865)
• Pander studied chick development
• studied the structures of the
undeveloped egg
• the early development of the
blastoderm
• the appearance of the primitive streak
with its two primitive lateral folds.
• described the beginning somites,
which he identified as rudimentary
vertebrae.
• Discovered the 3 germ layers –
ectoderm endoderm and mesoderm

15. KARL ERNST VON BAER (1792-1876)
• In 1827, he described the ovum in
ovarian follicles of a dog.
• In 1819 von Baer repeated Pander's
investigations on the chick
• identified the notochord as a distinct
and primary structure
• recognized that primitive ridges were
really neural folds that closed upon
each other and eventually marked
the spinal column

16. HEINRICH RATHKE (1793-1860)
• found time to undertake and publish detailed
embryological studies of vertebrates and the
crayfish
• produce important monographs on development
in fish, reptiles, and vertebrates in general.
• focus more on the details of individual
transformations
• followed development of vertebrate skull,
excretory system and respiratory system
• First to identify pharyngeal arches
• Showed that the same embryonic structure that
became gill supports in fishes became jaws and
ears in mammals

17. RECAPITULATION THEORY
• Ernest Haeckel and Fritz Muller
proposed it independently around the
same time.
• Also called Biogenetic Law
• States that ontogeny repeats
phylogeny
• Means developmental stages of higher
animals closely resemble the adults of
their ancestral species
Ernest Haeckel

18. FRITZ MULLER
• A naturalist and resident of Brazil
• monograph entitled Fur Darwin -
detailed a comparative study of the
embryology of crustaceans.
• described two possible patterns of
development :
1) embryos either diverge in form from
one another after traveling a pace on a
common developmental path
OR
2) the evolutionarily more advanced
embryos trace the entire
developmental path of their ancestors
to the adult stage and progress beyond.
Muller

19. MOSAIC THEORY
• By William Roux
• Explains cell division
• According to this each division actually
separates off differential nuclear materials
into the different daughter cells. The
process is rather like producing a mosaic,
in which each resulting piece is different
in that it has different bits of nuclear
material from the others though it
maintains its individuality.

20. GERMPLASM THEORY
• By Augustus Weismann in 1883
• States that the body is divided
into somatoplasm (somatic cells)
and germplasm (germ cells)
• The germ cells are not influenced
by the body bears them
• Somatic cells form from germ
cells

21. REGULATIVE THEORY
• Hans Driesch proposed in 1892
• Based on experiments in sea urchin
• After first cell division he shook the cells
apart and placed them separately.
• Instead of producing partial embryos
the cells produced normal embryos that
were half-sized.
• “Totipotency” : ability to respond to the
needs of the whole and to become any
part of the whole that the conditions
demanded

23. What is induction?
• Coordination in the construction of organs is accomplished by
one group of cells changing the behavior of an adjacent set of
cells, thereby causing them to change their shape, mitotic rate,
or fate.
• This kind of interaction at close range between two or more
cells or tissues of different history and properties is called
induction.
HOW DO THE CELLS KNOW THAT THEY ARE ADJACENT TO
A PARTICULAR CELL AND HAVE TO FORM A
PARTICLULAR ORGAN?

24. • The most celebrated of the induction experiments were described
by Hans Spemann and his student Hilde Mangold in 1924 – called
organizer effect.
• Nobel Prize was awarded to Hans Spemann in 1935 : discovery of
organizer effect in embryonic development.

25. EXPERIMENT
1) they removed cells from the dorsal lip of an amphibian
blastula
2) transplanted them to a different location (to belly
region)on another blastula
• The dorsal lip region - site of origin of those mesoderm
cells produce the notochord
RESULT: Embryo developed two notochords, a normal
dorsal one and a second one along its belly

27. • Spemann and Mangold used genetically different donor and
host blastula for and showed that notochord produced by
transplanting dorsal lip cells contained host cells as well as
transplanted ones.
• Transplanted dorsal lip cells – act as organizers of notochord
development
• These cells stimulated a developmental program (notochord
development) in the belly cells of the embryos in which they
were transplanted
• belly cells clearly contained this developmental program but
would not have expressed it in the normal course of their
development.

29. • The theory of development had to explain why eggs of different
species developed in different way
• Developmental mechanisms explained by Roux, Driesch etc
had genetic components. But back then it was just considered
embryology
• Split between embryology and genetics came mainly through
the works of Thomas Hunt Morgan.
• Most embryologists supported Boveri – Nucleus was the site of
hereditary determinants
• Morgan argued that hereditary determinants lay in cytoplasm –
Collaborated with Driesch- removal of cytoplasm from
uncleaved egg defective embryos formed.

30. • 1905 : E. B. Wilson and his student Nettie Stevens – discovery
that XX chromosome - female animals and the XO or XY
chromosome - male animals. So, nucleus determined the sex of
the individual.
• Morgan responded by investigating a parthenogenetic species
of aphids- correlating chromosome number and sex.
Interpreted results such that cytoplasm still determined
development
• 1910 : Morgan had found mutations in Drosophila - interpreted
as segregating with the X chromosome- initially resisted this
interpretation
• eventually came to see the genes as physically linked on the
chromosomes
• Thus, it lead to founding of the gene theory

31. • Slow process of progressive change – development
• Development of multicellular organisms begins with fertilized
egg (zygote), which divides mitotically to produce cells of the
body.
• The single cell, fertilized egg gives rise to different types of cells-
muscle cells, nerve cells, epidermal cells etc. This process is
called cell differentiation.

32. FERTILISATION
CLEAVAGE
GASTRULATION
ORGANOGENESIS
METAMORPHOSIS
GAMETOGENESIS

33. • Stages of development between fertilization and hatching are
called embryogenesis
• Fertilization: fusion of mature sex cells- sperm and egg
(gametes)
• Cleavage: series of extremely rapid mitotic division
immediately following fertilization. During this, zygote
cytoplasm divides into numerous small cells called
blastomeres.
• By the end of cleavage, blastomeres form a sphere called
blastula.
• Then the rate of mitosis slows down and the blastomeres move
and rearrange themselves. This process is called gastrulation.
The embryo is said to be in gastrula stage.

34. • As a result of gastrulation the three germ layers- ectoderm,
mesoderm and endoderm are established.
• The cells of the germ layers interact with one another and
rearrange themselves to produce tissues and organs. This
process is called organogenesis.
• Many organs contain cells from more than one germ layer. For
example – outer layer of skin (epidermis) : ectoderm
inner layer of skin(dermis) : mesoderm

35. METAMORPHOSIS
• In many organisms the young one born after hatching is not
sexually mature and needs to undergo metamorphosis to
become sexually mature.
• Under the control of molting and juvenile hormones
• Two types of metamorphosis : Holometamorphosis and
Hemimetamorphosis
• Holometabolous forms (complete metamorphosis): 4 stages-
embryo, larva, pupa, adult. Each stage differs completely from
the other. Eg: butterfly
• Hemimetabolous forms(incomplete metamorphosis): 3
stages - egg, nymph and adult. A type of gradual change in
which the nymph somewhat resembles the adult. Eg :
Grasshoppers

37. REFERENCES
• Browder, Leon W.; A Conceptual History of Modern
Embryology. 1st edition. Plenum Press. New York and
London.1991.
• Gilbert, Scott F. ; Developmental Biology. 9th edition. Sinauer
Associates, Inc. Massachusetts, USA. 2010.
• http://plato.stanford.edu/entries/biology-developmental
• https://embryo.asu.edu/pages/historyembryology1959joseph-
needham