Phylum Chordata Chordata • : — Protochordata – Triploblastic • Special features: – Bilateral – No – Eucoelomate – pharyngeal arches and slits → ciliated atrium – • Filter feeding & gas exchange • Special features: – Open – Metanephridia-type excretory system (misidentified as protonephridia in most texts) – Dorsal hollow tube and/or secretion into atrium – Post-anal • Classes: – → – Urochordata • mesodermal blocs – • pharyngeal arches and slits – Cephalochordata

Pelagic colonial

Solitary squirt Urochordates – (sea squirts)

Wild convergence! • like early development • Mollusk-like bauplan • Plant-like tunic • -like or gelatinous final Colonial

Larvaceans - • External suspension feeding • “house” • Reduced

Heyer 1 Chordata

Phylum Chordata Axial of a bony fish — subphylum Vertebrata (Craniata) : • Special features: Cranium & – Prolonged embryonic development – Strong cephalization • in cranium – mesodermal blocs →

• Myomeres & vertebrae – Closed circulatory system • Agnathans — jawless • Gnathostomes — jawed vertebrates • — axial skeleton only • — axial + appendicular skelton Figure 3.02

Axial & of Phylum Chordata

key Axial skeleton — subphylum Vertebrata (Craniata) Appendicular Cranium a bipedal skeleton • Classes: Pectoral girdle – — jawless fishes

Vertebra – — cartilaginous Pelvic fishes girdle – — bony fishes – Amphibia – Reptilia – Aves — – Mammalia Figure 49.26

Heyer 2 Systematics

⇐TET classes

Both systems have valid uses Fertilization of a mammalian

Vertebrate Development

Follicle cell

Sperm Zona Cortical ganules pellucida Egg plasma membrane Acrosomal vesicle EGG CYTOPLASM

From M.K. Richardson (1997) &

Animal hemisphere Radial & 47-8: Frog body polarity — Point of 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 Right Gray crescent

Ventral Dorsal First cleavage


Posterior Body axes Establishing the axes

SURFACE VIEW CROSS SECTION 47-9: Frog body polarity — Animal pole Cleavage planes 47-12: frog 0.25 mm

2-cell Dorsal stage tip of Dorsal lip blastopore forming of blastopore Vegetal pole Blastula

4-cell Blastocoel stage shrinking forming Eight-cell stage (viewed from the animal pole) 8-cell stage 0.25 mm

Ectoderm Blastocoel Animal pole Blasto- coel remnant Blastula (cross section) Key

Future Vegetal pole Future mesoderm Yolk plug Yolk plug Blastula (at least 128 cells) Future endoderm Gastrula

3 47-23a: frog fate map Neural Neural plate fold — unique to

Neural folds Central Epidermis


LM 1 mm

Neural Neural Mesoderm fold plate Outer layer of ectoderm Endoderm Neural crest Notochord Ectoderm Blastula 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

Eye Tail bud Development chordate segmentation

Mesoderm lateral to the notochord forms blocks

called somites SEM Neural tube 1 mm Lateral to the somites, the Notochord Neural mesoderm splits to form the crest coelom

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 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 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 () forms in blastoderm. • from germ layers. • All three germ layers form from infolding epiblast.

Eye Epiblast Neural tube Notochord Somite Coelom Future Archenteron Primitive ectoderm Lateral fold Endoderm streak Mesoderm vessels Ectoderm

Yolk stalk Somites Migrating 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 () Endometrium (uterine lining) 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 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.

Allantois Amnion

Chorion Ectoderm Mesoderm Endoderm

Yolk sac Extraembryonic mesoderm Gastrulation has produced a three-layered embryo with four 1 mm extraembryonic membranes.