Animal Development

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The frog blastula is many cell layers thick. Cells of the dorsal lip originate in the gray crescent and invaginate to create the archenteron.

Cells continue to move from the embryo surface into the embryo by involution. These cells become the endoderm and mesoderm.

The blastopore encircles a yolk plug when gastrulation is completed.

The surface of the embryo is now ectoderm, the innermost layer is endoderm, and the middle layer is mesoderm.

Slide 23

Gastrulation in a frog embryo

Future ectoderm

Key

Future endoderm

Future mesoderm

SURFACE VIEW

Animal pole

Vegetal pole

Early gastrula

Blastopore

Blastocoel

Dorsal lip of blasto- pore

CROSS SECTION

Dorsal lip of blastopore

Late gastrula

Blastocoel shrinking

Archenteron

Blastocoel remnant

Archenteron

Blastopore

Blastopore

Yolk plug

Ectoderm

Mesoderm

Endoderm

Slide 24

Gastrulation in the chick

The embryo forms from a blastoderm and sits on top of a large yolk mass.

During gastrulation, the upper layer of the blastoderm (epiblast) moves toward the midline of the blastoderm and then into the embryo toward the yolk.

The midline thickens and is called the primitive streak.

The movement of different epiblast cells gives rise to the endoderm, mesoderm, and ectoderm.

Slide 25

Gastrulation in a chick embryo

Endoderm

Future ectoderm

Migrating cells (mesoderm)

Hypoblast

Dorsal

Fertilized egg

Blastocoel

YOLK

Anterior

Right

Ventral

Posterior

Left

Epiblast

Primitive streak

Embryo

Yolk

Primitive streak

Slide 26

Organogenesis

During organogenesis, various regions of the germ layers develop into rudimentary organs.

The frog is used as a model for organogenesis.

Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm.

Slide 27

Early organogenesis in a frog embryo

Neural folds

Tail bud

Neural tube

(b) Neural tube formation

Neural fold

Neural plate

Neural fold

Neural plate

Neural crest cells

Neural crest cells

Outer layer of ectoderm

Mesoderm

Notochord

Archenteron

Ectoderm

Endoderm

(a) Neural plate formation

(c) Somites

Neural tube

Coelom

Notochord

1 mm

1 mm

SEM

Somite

Neural crest cells

Archenteron (digestive cavity)

Somites

Eye

Slide 28