1. The oocyte is swept out of the ovary and into the oviduct by fimbriae.
2. Fertilization occurs in the ampulla region (close to the ovary)
3. The cells divide to form a 16 cell morula and loose the zona peludica once inside the uterus.
4. The cells become a trophoblast and the inner cell mass
5. It implants into the endometrium (wall) of uterus ICM first.

The blastocyst forms the trophoblast and the ICM.

The ICM gives rise to the yolk sac and the epiblast.

The epiblast gives rise to the embryonic epiblast which forms the embryonic ectoderm and the primitive streak.

The primitive streak forms the embryonic endoderm and mesoderm.

• The ectodermal cells form the amniotic sac (hollow ball of cells) with epiblast cells at the bottom. These invaginate upwards to form the primitive streak and the primative node. (End of streak - node = caudal - rostral (orientation))
• The endodermal cells form the yolk sac (also a hollow ball of cells) with hypoblast cells at the top.
• Wedged inbetween the two sacs like a weird embryonic sandwich are the mesodermal cells.
• The epiblast, mesodermal cells and hypoblast form the bilaminar embryonic disk.

The chorion is the area underneath the wall of the uterus where the embryo gets nutrients- the umbilical chord branches off it.

The amnion is the amniotic sac that surrounds the embryo absorbic shock from surroundings.

1. Two blastocel and two ICM forms two chorions and two amnions. Non-identical twins.
2. One blastocel and two ICM forms one chorion and two amnions. Identical twins. They share the same source for their umbilical chords.
3. One blastocel and one ICM forms one chorion and one amnion. Identical twins (could be conjoined). They share the same source of their umbilical chords and the same amniotic sac.

That at the two celled and the four celled stages each individual cell has the ability to give rise to the entire embryonic pattern.

This is totipotency.

What did Horstadius' experiments reveal?

(at the first stage)

He separated the top four cells from the bottom four cells in an eight celled sea urchin embryo. The top four cells became ciliated blastula and the bottom four cells became a vegetalised blob (formed mainly of gut).

He showed that as development proceeds, blastomeres lose their totipotency and become commited to a particular development type.

What did Horstadius' experiments reveal?

(at the second stage)

He separated out the top four cells from the eight celled sea urchin embryo and they gave rise to ciliated blastulae.

He separated out the bottom four cells and they gave rise to vegetalized plutei.

He showed that blastomeres at the eight cell stage are no longer totipotent, but each of the top four or bottom four cells have the same developmental potential and are therefore equipotent (or an equivalence group).

Early embryos have capacity for regulation and have high developmental plasticity.

As they develop cells become more restricted in their developmental potential displaying commitment then differentiation.

How would we clone an animal?

What does the transfer of an egg to a different cytoplasm prove?

1. Apply UV radiation to the donor cell to destroy the DNA.
2. Remove the nucleus from another cell.
3. Implant the nuclei into the enucleated egg.

It proves that information is maintained throughout development and that a differentiated cell nucleus still has the genetic information to give rise to a whole organism.

Internal- asymetric distribution of factors cleave unevenly as cells divide, giving rise to different cytoplasmic properties

External- cells signal to one another

1. P granules localise to the germ line in C.elegans
2. Vg1 mRNA transcripts localise to the vegetal pole of the Xenopus egg cell

The animal-vegetal axis is determined maternally.

Before fertilization the egg had radial symetry.

The sperm entry point sets up the D/V axis- the dorsal end is placed opposite to the point of sperm entry.

Within the first 90 minutes after fertilization, the cortex roatates approximately 30°.

The vegetal cortex opposite the sperm entry point moves towards the animal pole.

The gray crescent forms in the space opposite the point of sperm enty in the area the the egg rotated.

When the cell is divided along the plane of the first cleavage into two blastomeres (each gets half of the gray crescent) each cell develops into a perfectly normal embryo.

When only one of the two blastomeres recieves the entire gray crescent, it alone forms a normal embryo. The other half produces a belly piece (ventral mesoderm).

Therefore the gray crescent must be inportant for signalling.