Phylum Chordata Phylum Chordata • Embryonic development: — subphylum Protochordata – Triploblastic • Special features: – Bilateral symmetry – No cephalization – Eucoelomate – pharyngeal arches and slits → ciliated atrium – Deuterostome • Filter feeding & gas exchange • Special features: – Open circulatory system – Notochord – Metanephridia-type excretory system (misidentified as protonephridia in most texts) – Dorsal hollow nerve tube and/or secretion into atrium – Post-anal tail • Classes: – Segmentation → – Urochordata • mesodermal blocs – Larvacea • pharyngeal arches and slits – Cephalochordata
Pelagic colonial salp
Heyer 1 Chordata
Phylum Chordata Axial skeleton of a bony ﬁsh — subphylum Vertebrata (Craniata) Axial skeleton: • Special features: Cranium & vertebral column – Prolonged embryonic development – Strong cephalization • Brain in cranium – mesodermal blocs →
• Myomeres & vertebrae Vertebra – Closed circulatory system • Agnathans — jawless vertebrates • Gnathostomes — jawed vertebrates • Fishes — axial skeleton only • Tetrapods — axial + appendicular skelton Figure 3.02
Axial & appendicular skeleton of Phylum Chordata
Heyer 2 Chordate Systematics
Both systems have valid uses Fertilization of a mammalian egg
Animal hemisphere Radial Cleavage & Blastulation — Frog 47-8: Frog body polarity — Point of Animal pole established during oogenesis sperm entry & fertilization • Large yolk content Vegetal necessitates hemisphere Vegetal pole asymmetrical blastulation Point of sperm entry Future dorsal Anterior side of tadpole Right Gray crescent
Ventral Dorsal First cleavage
Posterior Body axes Establishing the axes
2-cell Dorsal stage tip of Dorsal lip blastopore forming of blastopore Vegetal pole Blastula
4-cell Blastocoel stage shrinking Archenteron forming Eight-cell stage (viewed from the animal pole) 8-cell stage 0.25 mm
Future ectoderm Vegetal pole Future mesoderm Yolk plug Yolk plug Blastula (at least 128 cells) Future endoderm Gastrula
Neural folds Central Epidermis nervous system
Notochord Neural crest
LM 1 mm
Neural Neural Mesoderm fold plate Outer layer of ectoderm Endoderm Neural crest Notochord Ectoderm Blastula Neural tube stage Mesoderm Neural tube (transverse section) Endoderm Formation of the neural tube Archenteron Fate map of a frog embryo
47-14: frog Neural plate formation neurulation
Mesoderm lateral to the notochord forms blocks
Archenteron (digestive cavity)
47-14c: frog segmentation Somites Source: http://www.ucalgary.ca/UofC/eduweb/virtualembryo/why_ﬁsh.html
Disk of Another way — asymmetric Fertilized egg cytoplasm blastulation in many vertebrates 47-10: Chick cleavage Zygote
Fertilized egg Figure 47.10 • Large, yolk-rich eggs Disk of cytoplasm • Cleavage forms the Four-cell stage 1 Zygote. Most of the cell’s volume is yolk, blastoderm. with a small disk of cytoplasm located at the animal pole. • Separation of the 2 epiblast from the Four-cell stage. Blastoderm hypoblast forms the blastocoel. 3 Blastoderm. The many cleavage Cutaway view of divisions produce the blastoderm, a mass of the blastoderm cells that rests on top of the yolk mass. Blastocoel Cutaway view of the BLASTODERM Blastocoel blastoderm. The cells of the BLASTODERM blastoderm are arranged in two layers, the epiblast and YOLK MASS YOLK MASS hypoblast, that enclose a fluid- Epiblast Hypoblast Epiblast Hypoblast filled cavity, the blastocoel.
4 Gastrulation — Chick Gastrulation — Chick • Instead of blastopore, groove (primitive streak) forms in blastoderm. • Organogenesis from germ layers. • All three germ layers form from infolding epiblast.
Yolk stalk Somites Migrating Yolk sac Endoderm Form extraembryonic Neural tube cells membranes YOLK (mesoderm) Hypoblast (a) Early organogenesis. The archenteron forms when (b) Late organogenesis. 56 hours old lateral folds pinch the embryo away from the yolk. chick embryo, about 2–3 mm long (LM). YOLK
Figure 47.13 Figure 47.15
chick extra-embryonic membranes (amniote) Endometrium (uterine lining) Amnion Allantois 47-18a: mammalian Inner cell mass blastulation Embryo Trophoblast Blastocoel Amniotic Albumen Blastocyst cavity with reaches uterus. amniotic fluid Expanding region of Maternal trophoblast blood Shell vessel Epiblast
Yolk Hypoblast Chorion (nutrients) Trophoblast Yolk sac Blastocyst 47-17: chick extra-embryonic membranes implants.
6-week human embryo Expanding 47-18b : mammalian region of gastrulation trophoblast Amniotic cavity Amnion Epiblast Hypoblast Chorion (from trophoblast Yolk sac (from hypoblast) Extraembryonic membranes start Extraembryonic mesoderm cells to form and (from epiblast) gastrulation begins.
Chorion Ectoderm Mesoderm Endoderm
Yolk sac Extraembryonic mesoderm Gastrulation has produced a three-layered embryo with four 1 mm extraembryonic membranes.