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Life History and Embryology
of Vertebrates
Hazel Anne L. Tabo, MS
Study of development from fertilization to
The nature of relationship from: ancestor to
offspring can be studied.
It reveals ancestry.
Haeckel: “Ontogeny recapitulates phylogeny.”
(development repeats evolution).
De Beers: Recapitulation is not the only
relationship between embryos and ancestors.
Embryo (De Beers, 1930)
Embryonic structures may or may not be
present from their ancestors or
 Embryonic structures - vestigial (lost); or
retained to adulthood.
 Developmental stages may be repeated in
earlier stage but not in later stages.
 Developmental sequence may or may not
be altered in the descendant.
Gametes (sex cells, haploid chromosomes) union
to form diploid organism.
Gametes perform meiotic division  haploidy
Sperms (males) have various head appearance:
spherical, spatulate, hooked, lancet-shaped,
spiraled, capped (acrosome).
Sperm: Head (nucleus); Neck (mitochondria);
Flagella (movement).
Egg cells: Microlecithal (small yolk); Mesolecithal
(moderate yolk); Macrolecithal (abundant yolk).
Gametogenesis – formation of gametes
OOCYTE (egg cell; ovum)
Developed and released by ovarian follicle; vitelline
membrane (nourishes the egg)
In therian mammals: egg is enclosed in thick zona
pellucida, corona radiata, cells of ovarian follicle.
Other vertebrates: egg is enclosed after ovulation;
Jelly layers (amphibians); albumen (birds); horny,
membranous (fishes) or calcareous shells (reptiles,
Microlecithal: Amphioxus; Eutherian mammals
(placental nourishment)
Mesolecithal: Lampreys; some fishes; Amphibians
Macrolecithal: Most fishes; Reptiles; Birds;
Monotremes (egg-laying mammals)
In therian mammals: Penetration of sperm from
corona radiata to vitelline membrane of the egg.
Fertilization involves enzymatic and physical
interactions between sperm acrosome and egg
 Union of gametes: restores the diploid # of
 Activation of egg to refract additional sperm
 Zygote (fertilized egg)  Embryo (multicellular)
Blastula – structure from series of cell division
into multicellular (embryo).
Blastomeres – daughter cells
Blastocoel - coelom / cavity
Yolk – nutrition of developing embryo; process
of cleaving & blastula is dependent on the yolk
Morula (blastula) – multiple numbered cells
ready for uterine implantation.
Cleavage: 2-celled; 4-celled; 8-celled;
16-celled; 32-celled stages of blastula
Vertebrate blastula
Composed of single tissue layer with
hundred of cells.
 The cells exhibit cellular polarity related to
the axis of the adult body.
 Polarity: 1) animal pole - embryo
2) vegetal pole – yolk (gut)
 Blastocoel – body cavity
(animal pole)
(vegetal pole)
Blastula: Animal and vegetal poles
Animal pole – EPIBLAST or EPIMERE;
developing embryo nourished by the
vegetal pole (dorsal cells--ECTODERM)
 Vegetal pole – HYPOBLAST or HYPOMERE
or developing yolk; nourishes the embryo
 GUT (ventral cells--ENDODERM)
Microlecithal (Amphioxus) – “holoblastic” (total
cleavage furrows penetrate the entire yolk);
equal-sized blastomeres
Animal pole
Blastocoel or
Vegetal pole
Cleavage in Amphibians
Mesolecithal - holoblastic; unequal-sized
 Vegetal pole – larger blastomeres;
nourishes the embryo; slower
 Animal pole – smaller blastomeres;
developing embryo
 Blastocoel – towards animal pole
Cleavage in Birds (Aves)
Macrolecithal – “meroblastic” (partial
cleavage); unequal-sized blastomeres
 Vegetal pole – large size yolk mass; too
great to be penetrated by cleavage furrow
 Animal pole – is relatively small
 Narrow blastocoel
Birds: Yolk > Embryo
Cleavage in Mammals
Microlecithal - holoblastic;
unequal-sized blastomeres
 Blastula  Blastocyst
(inner cell mass formation)
 Trophoblast – superficial
layer of cells
 Blastocoele – towards
vegetal pole
 Embryoblast – animal
pole; epimeric cells
 Endometrium – inner
lining of the uterus (mother)
Cleavage patterns followed by holoblastic (total
cleavage) and meroblastic (partial cleavage) eggs
 Radial (sea urchin,
 Bilateral (tunicates,
 Spiral (annelids,
 Rotational (mammals)
 Discoidal (fish, birds,
 Superficial (insects)
Gastrulation: formation of three germ layers
Blastula  Gastrula
Animal pole – EPIBLAST or EPIMERE; developing
embryo (dorsal cells--ECTODERM)
Vegetal pole – HYPOBLAST or HYPOMERE or Yolk;
nourishes the embryo  GUT (ventral cells-ENDODERM)
Germ layers:
– 1) Ectoderm – from epiblast (animal pole); outermost
– 2) Mesoderm – middle layer; MESENCHYME
– 3) Endoderm – from hypoblast (vegetal pole);
innermost layer
(animal pole)
(vegetal pole)
Cells migrate to the interior of the blastula, consequently
forming two (in diploblastic animals) into three (triploblastic)
germ layers.
The embryo during this process is called a gastrula.
Among the different animals, different combinations of the
following processes occur to place the cells in the interior of
the embryo:
– Epiboly - expansion of one cell sheet over other cells
– Ingression - cells move with pseudopods
– Invagination - forming the mouth, anus, and archenteron
– Delamination - the external cells divide, leaving the
daughter cells in the cavity
– Polar proliferation
In most animals a blastopore is formed at
the point where cells are entering the
 Two major groups of animals can be
distinguished according to the blastopore's
fate. In deuterostomes the anus forms
from the blastopore (formerly the mouth),
while in protostomes it develops into the
Blastopore – an opening that resulted from invagination
of cells towards embryo
A. Protostomes
B. Deuterostomes
Holoblastic Gastrula: mesoderm
Vegetal (yolk) pole – folds inward towards animal
pole  double-walled cup
Cup opening – blastopore continues to another end
Gastrocoel – body cavity (coelom)
Ectoderm – uppermost layer of cells
Archenteron – primitive gut (yolk) lined by
Mesoderm – middle layer formed from dorsolateral
outpocketing of archenteron
Notochord – anlagen of nervous system (primitive),
formed from dorsal wall of archenteron
Archenteron (primitive gut)
Holoblastic gastrula
Notochord – anlagen of nervous system
(primitive), formed from dorsal wall of
 Chordamesoderm – roof of the archenteron,
forms the notochord in the midline and somites
(series of paired mesodermal tissue blocks)
 No outpocketing from the gut tube - schizocoel
 Coelom – formed by splitting of the hypoblast in
the somites (schizocoel – pouch form)
Mammalian embryo
Blastocoel – indirectly filled with maternal fluid
and enlarges  Blastocyst
Delamination of endoderm (hypoblast) from
inner cell mass and cover the trophoblast
Blastoderm – (or embryoblast) flattened inner
cell mass forms primitive streak (notochord 
Mesoderm – forms extraembryonic membranes
(fetal membranes) and body of embryo
(musculoskeletal and circulatory system);
Mesenchymal tissue
Coelom – forms by schizocoel (pouch form)
Embryoblast or Blastoderm
inner cell mass +
primitive streak
 Primitive streak –
(anlagen of CNS);
Germ layers
Ectoderm – Central nervous system, retina and
lens, cranial and sensory, ganglia and nerves,
pigment cells (melanocytes), head connective
tissue, epidermis of skin, hair, mammary glands.
Mesoderm – musculoskeletal system, circulatory
system, dermis of skin, connective tissue,
urogenital system, heart, blood (lymph cells), and
Endoderm – gastrointestinal system (gut);
stomach, colon, liver, pancreas, urinary bladder;
lining of urethra, epithelial parts of trachea, lungs,
pharynx, thyroid, parathyroid, intestine.
Overlaps with gastrulation establishing the central
nervous system (CNS)
Gastrula  Neurula
Neurocoel – coelom
Chordamesoderm – induces thickening of the
ectoderm into a neural plate
Neural crest cells – (Ectomesenchyme) considered
as the 4th germ layer which arised from
ECTODERM forms cartilage & bones of the
head, pharyngeal cartilages; peripheral nerve
ganglia, some glandular tissues; melanocytes
Neural plate – ectodermal cells
 Neural folds – formed from neural plate;
fold of cells that arches and meet at the
mid-dorsal line and forms the neural tube.
 Neural tube – encloses the neurocoel
 Neurocoel – anlagen of the dorsal hollow
nerve cord  (+) chordates
Migration of primordial germ cells
Holoblastic embryo: Establishes the head and tail
Meroblastic embryo: 3 germ layers spread faced
down on the uncleaved yolk
Embryo increase in length while the archenteron
becomes part of the gut tube
Schizocoely  Enterocoelom (blastopore becomes
anus while the mouth opens anteriorly.
Organogenesis: Endoderm
Endoderm form the lining of the gut.
Archenteron: Foregut, Midgut & Hindgut
Major Structures
Foregut Epithelium: oral cavity,
nasal cavity, pharynx, gill, esophagus, lung
Midgut Visceral organs: stomach, liver, gall bladder,
pancreas, intestines, germ cells of gonads
(ovary & testis), part of the yolk sac
membrane, part of the allantois
Hindgut Urinary bladder; cloaca / anus
Organogenesis: Mesoderm
Mesoderm layer
Major structure
Outer Epimere – Dermatome Skin Dermis
Middle Epimere – Myotome
Muscles: axial, appendicular, branchiomeric,
Inner Epimere – Sclerotome
Vertebral column
Intermediate mesoderm
Kidney; Urogenital ducts
Somatic hypomere
Bones: ribs, sternum (breast plate),
appendicular skeleton, some appendicular
muscle, parietal peritoneum (outer abdominal
wall); genital ridge; part of amnion and chorion
Splanchnic hypomere
Blood, heart, gut, smooth muscle, visceral
peritoneum (inner abdominal wall); yolk sac
and allantois
Organogenesis: Ectoderm
Ectoderm layer
Major structure
Somatic ectoderm
Skin epidermis; Teeth enamel; stomodeum
(mouth); proctodeum (anus/cloaca); gill
epithelium; part of amnion and chorion
Neural plate ectoderm
Brain; Spinal cord
Epidermal placodes
Capsules: olfactory (nasal); optic (eye); otic
(ear); epibranchial capsules; neuromast
(electroreceptors organs); ganglia of some
cranial nerves
(Neural crest)
Spinal ganglia; splanchnocranium;
neurocranium (part); dermatocranium (part);
teeth dentine; eye cornea; chromatophores
(pigment cells); branchiomeric muscles
(part); aortic arches; heart septum (divides
the right and left side of heart)
Extraembryonic membranes (fetal
membranes) and Placenta
Amphibians – yolk cleaves and directly
incorporated into somatic cells (body cells)
 Yolk supply – limited
 Early hatching  Larva
 No fetal membranes required to sustain
Yolk remains uncleaved, eggs are laid in
water; yolk sac is the only fetal membrane
 Yolk sac – formed from 3 germ layers to
absorb the yolk into the body
 Respiration and excretion are direct
contact with the environment
Reptiles and Birds (Avians)
Primitive streak  extraembryonic mesoderm splits
 forms extraembryonic coelom 
Splanchnopleure + adjacent Yolk = Yolk sac
Allantois (vascular) – under the shell; near the
chorioallantoic membrane. Allantois (respiration,
receives excretory wastes; absorbs albumen; takes
some minerals from the shell)
Somatopleure + head fold of amnion = Chorion
(outer) and Amnion (inner)
Amnion – “water bag” contains amniotic fluid that
bathes the embryo; cushions the embryo (shelter
for growth)
Eutherian mammals
Placental mammals – nourish their young
in the uterus, exchange and nutrition of
fetus occurs between fetal and maternal
bloodstream by PLACENTA
 Yolk sac and Allantois – vascularized
(blood vessels)  supplies nutrition to
 Chorion and Amnion – avascular (no blood
vessels)  cannot support placenta
Allantoic circulation (birds & reptiles) –
homologous to umbilical circulation (mammals)
 Amnion – sac filled with amniotic fluid bathes
the fetus; allows the early fetus to move freely
and protects the fetus from pressure of the
maternal abdomen
 Amniotes – collective terms for reptiles, birds
and mammals because the possess amnion.
 Maternal contribution: Chorion (lies between
allantois & uterus) incorporated into the
placenta  CHORIOALLANTOIC Membrane
 Fetal contribution: Allantois (in most mammals)
 as fetus grows, allantois decreases in size
Prototherians – egg-laying mammals;
deposits egg in a pouch (oviparous)
 Metatherians – marsupials (pouched
animals) – no typical placenta
 Eutherians – placental mammals