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Chordata
Phylum Chordata
•  Embryonic development:
–  Triploblastic
Phylum Chordata
— subphylum Protochordata
•  Special features:
–  No cephalization
–  Bilateral symmetry
–  Eucoelomate
–  pharyngeal arches and slits → ciliated atrium
•  Filter feeding & gas exchange
–  Deuterostome
•  Special features:
–  Notochord
–  Dorsal hollow nerve tube
–  Post-anal tail
–  Segmentation →
–  Open circulatory system
–  Metanephridia-type excretory system
(misidentified as protonephridia in most texts)
and/or secretion into atrium
•  Classes:
•  mesodermal blocs
•  pharyngeal arches and slits
–  Urochordata
–  Larvacea
–  Cephalochordata
Pelagic colonial salp
Solitary sea squirt
Urochordates – tunicates (sea squirts)
Wild convergence!
•  Vertebrate like early
development
•  Mollusk-like bauplan
•  Plant-like tunic
•  Sponge-like or gelatinous
final morphology
Colonial tunicate
Larvaceans
Cephalochordates - lancelets
•  External suspension feeding
•  Mucus “house”
•  Reduced pharynx
Heyer
1
Chordata
Phylum Chordata
— subphylum Vertebrata (Craniata)
•  Special features:
Axial skeleton of a bony fish
Axial skeleton:
Cranium & vertebral column
–  Prolonged embryonic development
–  Strong cephalization
• Brain in cranium
–  mesodermal blocs →
• Myomeres & vertebrae
–  Closed circulatory system
Vertebra
•  Agnathans — jawless vertebrates
•  Gnathostomes — jawed vertebrates
•  Fishes — axial skeleton only
•  Tetrapods — axial + appendicular skelton
Axial & appendicular skeleton of
a bipedal tetrapod
key
Axial skeleton
Appendicular
skeleton
Cranium
Figure 3.02
Phylum Chordata
— subphylum Vertebrata (Craniata)
•  Classes:
Pectoral
girdle
– Agnatha — jawless fishes
Vertebra
– Chondrichthyes — cartilaginous
fishes
Pelvic
girdle
– Osteichthyes — bony fishes
– Amphibia
– Reptilia
– Aves — birds
– Mammalia
Figure 49.26
Heyer
2
clades
Chordate Systematics
⇐TET classes
Both systems
have valid
uses
Fertilization of a mammalian egg
Vertebrate
Development
Follicle
cell
Sperm
Cortical
basal
ganules
body Sperm
nucleus
Zona
pellucida
Egg plasma
membrane
Acrosomal
vesicle
From
EGG CYTOPLASM
M.K. Richardson (1997) Anatomy & Embryology
Radial Cleavage & Blastulation — Frog
Point of
sperm entry
47-8: Frog body polarity —
established during oogenesis
& fertilization
• Large yolk
content
necessitates
asymmetrical
blastulation
Animal
hemisphere
Vegetal
hemisphere
Vegetal pole
Point of
sperm
entry
Future
dorsal
side of
tadpole
Anterior
Gray
crescent
Right
Ventral
Animal pole
First
cleavage
Dorsal
Left
Posterior
Body axes
Establishing the axes
CROSS SECTION
SURFACE VIEW
47-9: Frog body polarity —
Animal pole
Zygote
47-12: frog gastrulation
Cleavage planes
Blastocoel
0.25 mm
2-cell
stage
forming
Vegetal pole
4-cell
stage
forming
Dorsal lip
of blastopore
Dorsal
tip of
blastopore
Blastula
Blastocoel
shrinking
Archenteron
Eight-cell stage (viewed
from the animal pole)
8-cell
stage
0.25 mm
Ectoderm
Animal pole
Blastula
(cross
section)
Blastocoel
Mesoderm
Endoderm
Blastocoel
remnant
Key
Vegetal pole
Blastula (at least 128 cells)
Future ectoderm
Future mesoderm
Future endoderm
Yolk plug Yolk plug
Gastrula
3
Neurulation
Neural
fold
— unique to chordates
47-23a: frog fate map
Neural plate
Epidermis
Neural folds
Central
nervous
system
Epidermis
Notochord
Neural crest
LM
1 mm
Neural
fold
Mesoderm
Neural
plate
Outer layer
of ectoderm
Endoderm
Neural crest
Notochord
Ectoderm
Mesoderm
Formation of the neural tube
Fate map of a frog embryo
Archenteron
47-14: frog
Neural tube stage
(transverse section)
Blastula
Neural tube
Endoderm
Neural plate formation
neurulation
Somites
Eye
Fish Development
Tail bud
chordate segmentation
Mesoderm lateral to the
notochord forms blocks
called somites
Lateral to the somites, the
mesoderm splits to form the
coelom
SEM
Neural tube
Notochord
1 mm
Neural
crest
Coelom
Somite
Archenteron
(digestive cavity)
Somites
47-14c: frog segmentation
Source: http://www.ucalgary.ca/UofC/eduweb/virtualembryo/why_fish.html
Another way — asymmetric
blastulation in many vertebrates
• Large, yolk-rich eggs
• Cleavage forms the
blastoderm.
• Separation of the
epiblast from the
hypoblast forms the
blastocoel.
Fertilized egg
Fertilized egg
Disk of
cytoplasm
Zygote
47-10: Chick cleavage
Figure 47.10
Disk of
cytoplasm
Four-cell stage
1 Zygote. Most of the cell’s volume is yolk,
with a small disk of cytoplasm located at the
animal pole.
2
Four-cell stage.
Blastoderm
Cutaway view of
the blastoderm
3 Blastoderm. The many cleavage
divisions produce the blastoderm, a mass of
cells that rests on top of the yolk mass.
BLASTODERM
YOLK MASS
Epiblast
Blastocoel
Hypoblast
Cutaway view of the
blastoderm. The cells of the
blastoderm are arranged in two
layers, the epiblast and
hypoblast, that enclose a fluidfilled cavity, the blastocoel.
Blastocoel
BLASTODERM
YOLK MASS
Epiblast
Hypoblast
4
Gastrulation — Chick
Gastrulation — Chick
• Instead of blastopore, groove (primitive streak) forms in blastoderm.
• All three germ layers form from infolding epiblast.
epiblast.
• Organogenesis from germ layers.
Eye
Epiblast
Forebrain
Neural tube
Notochord
Somite
Heart
Coelom
Future
ectoderm
Archenteron
Primitive
streak
Endoderm
Mesoderm
Lateral fold
Blood
vessels
Ectoderm
Yolk stalk
Migrating
cells
(mesoderm)
Form extraembryonic
membranes
Endoderm
Hypoblast
Somites
Yolk sac
YOLK
(a) Early organogenesis. The archenteron forms when
lateral folds pinch the embryo away from the yolk.
YOLK
Figure 47.13
Neural tube
(b) Late organogenesis. 56 hours old
chick embryo, about 2–3 mm long (LM).
Figure 47.15
chick extra-embryonic membranes (amniote)
Amnion
Endometrium
(uterine lining)
Allantois
47-18a: mammalian
blastulation
Inner cell mass
Trophoblast
Embryo
Blastocoel
Amniotic
cavity
with
amniotic
fluid
Albumen
Blastocyst
reaches uterus.
Maternal
blood
vessel
Shell
Yolk
(nutrients)
Chorion
Yolk sac
47-17: chick extra-embryonic membranes
Expanding
region of
trophoblast
47-18b : mammalian
gastrulation
Expanding
region of
trophoblast
Epiblast
Hypoblast
Trophoblast
Blastocyst
implants.
6-week human embryo
Amniotic
cavity
Amnion
Epiblast
Hypoblast
Chorion (from
trophoblast
Yolk sac (from
hypoblast)
Extraembryonic
membranes start
to form and
gastrulation
begins.
Extraembryonic mesoderm cells
(from epiblast)
Allantois
Amnion
Chorion
Ectoderm
Mesoderm
Endoderm
Yolk sac
Extraembryonic
mesoderm
Gastrulation has produced a
three-layered embryo with four
extraembryonic membranes.
1 mm
5