-          Cleavage: -   

(5 facts)     

1. embryo divides repeatedly

2. large cytoplasmic mass converted into small cells (blastomeres)

3. no cell growth occurs, only subdivision until cells reach regular somatic cell size

4. at the end of cleavage, zygot has been divided into many 5. hundreds or thousands of cells;

5. blastula (blastocyst) is formed

when cleavage creates a cluster of cells

in mammals, called a blastocycst

What is polyspermy?

The entry (penetration) of more than one sperm; devastating to the organism

What mechanisms in sea urchins and mammals prevent polyspermy?

(2 facts)

1. Important changes in the egg surface block entrance to any additional sperm

2. In sea urchins, an electric potential rapidly spreads across the membrane, called “fast block”

Describe the animal-vegetal axis.

(3 facts)

1. Yolk is present at only one end, this is the Vegetal pole which establishes polarity in the embryo

2. The region containing mostly cytoplasm is the Animal pole

3. Cleavage affected by quantity and distribution of yolk present and also genes controlling the symmetry of cleavage

isolecithal eggs

(3 facts)(5 EXs)

1. very little yolk, distributed evenly in the cytoplasm

2. cleavage is holoblastic (cleavage furrow extends completely through egg)

3.Echinoderms, tunicates, cephalochordates, molluscs, and mammals

mesolecithal eggs

(3 facts) (1 EX)

1. moderate amount of yolk, concentrated in the vegetal pole

2. eggs cleave holoblastically, but cleavage is slowed in yolk-rich vegetal pole

3. occur in most Amphibians

telolecithal eggs

(4 facts)(4 EXs)

1. much yolk, concentrated at vegetal pole

2. cleavage is meroblastic (cleavage furrow does not cut through the heavy yolk; cleavage is confined to a narrow disc on top of the yolk)

3.  occur in Birds, reptiles, most fishes, and a few amphibians

4. young exhibit direct development

Tissues that arise from Ectoderm:

(2 EXs)

Outer epithelium of body and derivatives

1. EX: Neural tube

2. EX:Neural crest

Tissues that arise from Mesoderm:

(4 EXs)

1. EX: Notochord

2. EX: Lining of thoracic and abdominal cavities

3. EX: Circulatory system

4. EX:  Somites

4. EX: Organs of urogenital system

 Tissues that arise from  Endoderm:

(6 EXs)

1. Epithelium of respiratory tract

2. Pharynx

3. Epithelium of the gut

4. Liver, pancreas

5. Inner lining of urinary bladder

6. Smooth muscle and connective tissue of digestive tract

Which animals are protostomes?

(6 EXs)

1. Arthropods

2. Nematods

3. Annelids

4. Molluscs

5. Flatworms

6. Lophorphorates

Which animals are Deuterostomes?

(2 EXs)

1. Echinoderms

2. Chordates

What characters determine if protostome or deuterostome?

(4 EXs)

1. Radial or spiral cleavage

2. Regulative or mosaic development

3. Fate of blastopore to become mouth or anus

4. Coelom formation (enterocoely or schizocoely)

What is enterocoely?

What is the fate of the blastopore in (1)deuterostomes and (2) Protostomes?

1. Becomes anus
2. Becomes mouth 

What is a mosaic embryo (Protostome embryos)?

(4 facts)

1. Cell fate determined by distribution of morphogenetic determinants, in the egg cytoplasm

2. If separated, individual blastomeres not capable of directing development of normal embryo

3. Blastopore becomes the mouth, anus forms from a second opening

4. Coelom is formed by schizocoely

What is a Regulative embryo (Deuterostome embryos)?

(2 facts)

1. Fate of a cell depends on its interactions with neighboring cells, rather than on what piece of cytoplasm it acquired during cleavage

2. Blastopore becomes the anus, second opening becomes the mouth