DEVELOPMENT

this lab focuses on two major varieties of development, exemplified by amphibians, and birds.

- echinoderms develop from eggs that have very little yolk. - amphibians develop from eggs with a moderate amount of yolk. - birds develop from eggs with huge amounts of yolk.

the way early development progresses, particularly with respect to cleavage patterns and to morphogenesis.

have considerably more yolk in them than echinoderm eggs, and this yolk is more DENSE and more CONCENTRATED in the vegetal hemisphere than in the animal hemisphere. - Most amphibian eggs also have a pigmented animal half, and a non-pigmented vegetal half, a further manifestation of the egg's animal-vegetal polarity! *gray crescent (crescent of grayish hue) dorsal region

it has an effect on the process of cleavage, the morphology of the blastula, and the way that glastrulation occurs.

cleavage begins at the animal pole and proceeds to the vegetal pole.

- In sea urchins the nucleus is offset toward the animal pole. - In amphibians, in addition to the nucleus being offset toward the animal pole, there is more dense yolk in the vegetal hemisphere than in the animal hemisphere, and there is pigment on the surface of the animal hemisphere. - In chickens the nucleus is offset toward the animal pole, and there is very dense yolk in the vegetal hemisphere.

- The yolk affects cleavage. In amphibians, the yolk retards cleavage as it proceeds toward the vegetal pole. In fact, the second cleavage begins in the animal hemisphere before the first is complete in the vegetal half, and the third (horizontal) cleavage is closer to the animal than to the vegetal pole (perhaps where there is less resistance to the formation of a cleavage furrow), yielding animal blastomeres that are smaller than the vegetal blastomeres. Later cleavage is more rapid in the animal hemisphere than in the vegetal, so that animal hemisphere blastomeres continue to be smaller than vegetal blastomeres. - In chickens, there is a very large amount of yolk in eggs, so much that only a very small portion of the cell at the animal pole, the blastodisc, is free enough from yolk to have any cleavage occur in it at all.

- In echinoderms cleavage is holoblastic. - In amphibians cleavage is holoblastic, but slow at the vegetal pole. - In chickens cleavage is restricted to the animal pole. Cleavage of the chick zygote is quite different from that in echinoderms and amphibians. The first several cleavages are vertical, and extend only a very short distance downward; horizontal cleavages eventually occur, which give rise to several layers of blastomeres that make up a disc of cells situated on top of the mass of yolk below.

In echinoderms, the wall of the blastula is one-cell thick all around; the blastocoel forms so that it occupies the animal and vegetal hemispheres essentially equally. In amphibians, the floor of the blastocoel is considerably thicker than the roof of the blastocoel. The blastocoel forms essentially exclusively in the animal hemisphere of the blastula. In birds, the blastocoel is a very narrow space underlying the blastodisc, sitting on an enormous amount of yolk.

- Echinoderm gastrulation is like the classic pushing in the leaky tennis ball: an invagination from the vegetal pole into the blastocoel. - Amphibian gastrulation is an involution of cells just ventral to where the gray crescent was located in the zygote; this inward movement continues as cells of the animal hemisphere migrate down over the larger yolky cells of the vegetal hemisphere. - Bird gastrulation is in two steps: a lower layer of blastodisc cells migrates away, forming a layer of hypoblast cells below the epiblast (which the remaining cells of the blastodisc are now known as). Then, as the hypoblast expands, the epiblast cells migrate rearwards and medially to form the primitive streak, through which streak cells migrate to form the mesoblast, a layer of cell between the epiblast and the hypoblast.

- Ectoderm: epidermis of the skin, hair, nails, lens, cornea, entire nervous system; lining of the nose and mouth. - Mesoderm: muscle, bone, cartilage, notochord, blood, kidneys, gonads. - Endoderm: Lining of esophagus, stomach, intestine; lungs, liver, pancreas, thyroid, parathyroids, bladder lining.

The nucleus is away from the bottom and close to the top, the nucleus is offset toward the animal pole.

Has very distinct animal vegital polarity, have more yolk that is concentrated in the vegital hemisphere than in the animal hemisphere... the nucleus is also offset towards the vegital pole. no cell division in the animal pole.

small portion of cell is at animal polarization of yolk.

Having less yolk allows the eggs to develop quicker. Echinoderms develop the quickest, then frogs and then amphibians.

They all have a hollow space and develop the notochord, The hollow space is different size and shape in each blastocoel. All 3 of them have a blastocoel, but the amount and the location of the yolk is different for all of them where they actually are.

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

BLASTOCOEL

thickened region of the epiblast