Animal Development

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3

The body axes and their establishment in an amphibian

(a) The three axes of the fully

developed embryo

(b) Establishing the axes

Pigmented cortex

Right

First cleavage

Dorsal

Left

Posterior

Ventral

Anterior

Gray crescent

Future dorsal side

Vegetal hemisphere

Vegetal pole - yolk

Animal pole

Animal hemisphere

Point of sperm nucleus entry

Slide 17

Cleavage planes usually follow a pattern that is relative to the zygote’s animal and vegetal poles.

Cell division is slowed by yolk. Yolk can cause uneven cell division at the poles.

Holoblastic cleavage, complete division of the egg, occurs in species whose eggs have little or moderate amounts of yolk, such as sea urchins and frogs.

Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds.

Slide 18

Cleavage in a frog embryo

Blastula (cross section)

Blastocoel

Animal pole

4-cell stage forming

2-cell stage forming

Zygote

8-cell stage

Vegetal pole:

yolk

0.25 mm

0.25 mm

Slide 19

Gastrulation

Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut.

The three layers produced by gastrulation are called embryonic germ layers:

The ectoderm forms the outer layer

The endoderm lines the digestive tract

The mesoderm partly fills the space between the endoderm and ectoderm.

Slide 20

The blastula consists of a single layer of cells surrounding the blastocoel.

Mesenchyme cells migrate from the vegetal pole into the blastocoel.

The vegetal plate forms from the remaining cells of the vegetal pole and buckles inward through invagination.

The newly formed cavity is called the archenteron.

This opens through the blastopore, which will become the anus.

Gastrulation in the sea urchin embryo:

Slide 21

Gastrulation in a sea urchin embryo

Future ectoderm

Key

Future endoderm

Digestive tube (endoderm)

Mouth

Ectoderm

Mesenchyme (mesoderm forms future skeleton)

Anus (from blastopore)

Future mesoderm

Blastocoel

Archenteron - cavity

Blastopore

Blastopore

Mesenchyme cells

Blastocoel

Blastocoel

Mesenchyme cells

Vegetal Pole

Invagination

Vegetal plate

Vegetal pole

Animal pole

Filopodia pulling archenteron tip

50 µm

Slide 22

Gastrulation in the frog