Purdue Bio 111: Development of Amphibians and Birds Lab

Amount of yolk in echinoderm eggs

very little

Amount of yolk in amphibian eggs

moderate, more dense, more concentrated in vegetal hemisphere

Amount of yolk in bird eggs

huge

How is an amphibian egg's animal-vegetal polarity shown?

nucleus offset towards animal pole, animal-vegetal polarization of yolk (pigmented animal half, non-pigmented vegetal half)

gray crescent

formed after fertilization at what will ultimately be the dorsal region of the embryo

Where does amphibian cleavage begin?

animal pole (proceeds to vegetal pole)

What happens as the cleavage furrow in amphibians plows through the zygote's cytoplasm?

noticeable retardation of its progress as it makes its way through the vegetal cytoplasm, where there's a greater concentration of yolk

Second cleavage in amphibians

vertical and perpendicular to first cleavage (animal to vegetal)

Third cleavage in amphibians

horizontal, offset towards animal pole (upper tier of blastomeres is smaller than lower one)

Where does cleavage continue more rapidly in amphibians?

animal hemisphere (the blastomeres in animal hemisphere are smaller than vegetal ones)

What is eventually formed after enough cleavage divisions in amphibians?

blastula (blastocoel only in animal hemisphere, vegetal hemisphere is solid with large, yolk-packed cells)

Gastrulation in amphibians

cells at the ventral margin of where the gray crescent had been in the zygote begin to migrate into the interior of the embryo, producing a blastopore and archenteron that form from the side of the blastula rather than at its vegetal pole

What happens as gastrulation proceeds in amphibians?

blastocoel is gradually reduced in volume by the invaginating mesoderm and endoderm cells

What forms after gastrulation in amphibians?

three germ layers (endoderm, ectoderm, mesoderm)

endoderm

innermost germ layer; develops into the linings of the digestive tract and much of the respiratory system

ectoderm

the outer germ layer that develops into skin and nervous tissue

mesoderm

middle germ layer; develops into muscles, and much of the circulatory, reproductive, and excretory systems

How does the central nervous system begin?

thickening layer of cells that overlie the upper cells of the archenteron (archenteron roof); this neural plate folds into a neural tube that becomes the brain anteriorly, and the spinal cord

blastodisc (birds)

cleavages only occur in blastula that consists of flattened, disk-shaped region that sits on top of yolk at animal pole; leads to very distinct animal-vegetal polarity

First several cleavages in chick zygote

vertical, extend only a short distance downward

Horizontal cleavages in chick zygote

eventually occur to give rise to several layers of blastomeres

Gastrulation in Birds

initiated by the separation of cells from the lower surface of this blastoderm, forming a flimsy layer of endoderm (hypoblast) below the epiblast

blastocoel in birds

space between epiblast and hypoblast

Hypoblast growth in birds

grows to form a complete layer under epiblast; migration of cells of the epiblast towards its posterior and central region to form a primitive streak

primitive streak (birds)

thickened region of the epiblast

As cells migrate inward in birds, what happens?

primitive streak regresses, leaving the notochord and lateral mesoderm behind

neural plate in birds and frogs

forms as the ectoderm above the notochord thickens

neural tube in birds and frogs

neural plate folds to form it; forms the brain at its broad anterior end and the spinal cord at its narrower posterior end

optic vesicles

out-pockets in the anterio-lateral region of the brain that become the eyes

cerebral vesicles

developed from anterior region; later develop into the cerebral hemispheres of the brain

somites

blocks of mesoderm that form segmental structures like vertebrae

Where does the bird heart develop from?

a tube that forms the lateral mesoderm; it grows, twists, and loops to form a 4-chambered heart

What do the blastulas of echinoderms, frogs, and chickens have in common?

all three have a blastocoel in the blastula; location of the blastocoel varies