Embroyonic Development and Larvae

**vegetal pole**

The yolky, lower portion of the egg cell

**animal pole**

less yolky top end of the egg cell

**polyspermy**

If an egg is near where sperm are being released, there can be so many sperm cells nearby that the egg gets fertilized by more than one sperm

**polyploidy**

If an egg cell receives DNA from more than one sperm cell, the result

**first cleavage**

splits the zygote into two roughly-identical daughter cells.

**first cleavage plane**

his first cell division occurs along a plane containing the sperm entry point and the animal-vegetal axis

 **2-cell stage**

After 1st cleavage, the embryo contains 2-cells and is said to be at

**second cleavage**

* occurs on a plane that also contains the animal-vegetal axis, but this cleavage plane is oriented approximately 90-degrees to the first cleavage plane.

**4-cell stage**

2nd cleavage cuts each of the initial 2 cells in half, resulting in the

**third cleavage (t**he embryo is said to have reached the**8-cell stage)**

 plane generally occurs at right angles to both of the first two cleavage planes

 **Radial cleavage**

each of the 4 cells in the “northern hemisphere” sits directly above (i.e., due north of) one of the 4 cells in the southern hemisphere.

**Spiral cleavage**

the northern hemisphere rotates (usually westward) 45 degrees relative to the southern hemisphere, so that the 4 northern cells come to rest in the furrows between the 4 southern hemisphere cells

**mosaic development**

the developmental fate of the daughter cells formed by cleavage is completely determined from the moment of cleavage and cannot be changed after that

**regulative development**

the cells of the early embryo can (for a short time and under certain circumstances) shift roles and become different tissues or organs than they normally would have become. This enables the embryo to compensate for loss or damage to one of the cells and is called

**blastula**

When cleavage is finished, or nearly so, the resulting embryonic stage is called the

**epithelial layer**

Cells on the outer surface of the blastula form an

**blastocoel**

The fluid-filled, hollow space inside the blastula is called the

Gastrulation

Gastrulation is an important developmental process that occurs in all animals except sponges.

Endoderm

 The epithelial layer that has been folded into the interior of the embryo and now lines the gut

**gastroderm**

The epithelial layer that has been folded into the interior of the embryo and now lines the gut is called the **endoderm** (“inner skin”), except in cnidarians, where it’s called the

Protostomes

mouth forms first

deuterostomes

mouth forms second

 **coelom**

If that body cavity is lined with mesodermal tissue

**schizocoely**.

If the coelom forms by a split in the mesoderm, the process is called

**Enterocoely.** 

If the coelom forms from mesodermal outpocketings from the side of the gut, the process is called

**pseudocoelom**

If the body cavity is not lined with mesoderm, but instead is the remains of the early embryonic blastocoel, then the body cavity is called a

**acoelomate**

If the animal has neither a true coelom nor a pseudocoelom and is, instead, just solid tissue between the gut and outer surface, then it is described as being