1. Animal Development
Process of development from a single cell to an entire multi-cellular organism

2. Animal Development Fertilization Cleavage Gametogenesis Gastrulation
Organogenesis
Neurulation

3. 1: Fertilization
Haploid egg and haploid sperm combine to form a diploid zygote.
Eggs have 2 ways to block polyspermy
Fast Block: Change in the electrical potential of the egg plasma membrane
Slow Block: Creation of a ‘fertilization envelope’ that acts as a physical barrier keeping other sperm away from the fertilized egg

4. Vitelline Layer
Plasma Membrane 1n

5. Fast Block to Polyspermy1n

6. 1: One sperm penetrates the egg

7. 2: The plasma membrane changes its permeability and Na+ ions flood into the egg

8. 3: The egg is now (+)-charged and repels the (+)-charged proteins on other sperm surfaces
Na+
Na+
Na+ 1n 1n +
Na+
Na+ +
Na+

9. Slow Block to Polyspermy1n

10. 1: One sperm penetrates the egg

11. 2: Cortical vesicles inside the egg fuse with the plasma membrane

12. 2: The vesicles release enzymes into the space between the vitelline and plasma membranes

13. 3: The enzymes break the bonds between the vitelline and the plasma membranes. Water floods the space between the two layers and the vitelline layer rises. 1n 1n

14. 4: The vitelline layer hardens and becomes impermeable to sperm, thus forming the “fertilization envelope”. 1n 1n

15. The egg on the left is unfertilized, and has no envelope.
In eggs like the sea urchin and starfish, the fertilization envelopes are visible as a halo around the egg.
The egg on the left is unfertilized, and has no envelope.
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16. 2: Cleavage & Blastulation
Egg and sperm fuse to create a diploid single-celled zygote.
The zygote goes through many rapid MITOTIC cell divisions resulting in a ball (or disc) of cells called a blastula
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17. The Blastula
Through rapid divisions, the zygote develops into a blastula, a hollow ball (or disc) of cells
The individual cells are referred to as blastomeres
The empty space in the center is referred to as a blastocoel

18. Egg Type vs. Pattern of Cleavage
Early development is affected by the amount and distribution of yolk in an egg
Eggs with small amounts of evenly distributed yolk are called ISOLECITHAL
Isolecithal eggs divide evenly through Holoblastic Cleavage

19. Egg Type vs. Pattern of Cleavage
Eggs with large amounts of yolk concentrated at one end of the egg are called TELOLECITHAL.
MODERATELY telolecithal eggs divide by Holoblastic Cleavage, but the size of the cells differs between the two poles as does the placement of the blastocoel.

20. Holoblastic Cleavage in an Isolecithal egg vs
Holoblastic Cleavage in an Isolecithal egg vs. a Moderately Telolethical egg
Adapted from

21. Meroblastic Cleavage
Strongly Telolechithal eggs, like chicken eggs, have a large amount of yolk.
The nucleus is surrounded by a very small area devoid of yolk called the Active Cytoplasm.
These eggs divide by Meroblastic Cleavage: only the Active Cytoplasm divides and the blastula forms as a disc resting on the surface of the yolk.

22. Moderately Telolecithal Strongly Telolecithal
Holoblastic
Meroblastic
Moderately Telolecithal
Strongly Telolecithal
Isolecithal

23. 3: Gastrulation Beginning of cell differentiation
Characterized by cell migration, rather than division
Results in a Gastrula which will eventually develop into the digestive tract of the organism
The Gastrula is made up of 3 embryonic (or, germ) layers called the Endoderm, Ectoderm and Mesoderm.

24. 3: Gastrulation Blastocoel
1)The surface cells of the Blastula begin to move into the inside of embryo through a process called INVOLUTION.
Blastocoel

25. 3: Gastrulation Remnant of Blastocoel
2)The inverted cells push into the blastocoel forming a new internal cavity, the ARCHENTERON
Remnant of Blastocoel

26. 3: Gastrulation Remnant of Blastocoel BLASTOPORE
2)The inverted cells push into the blastocoel forming a new internal cavity, the ARCHENTERON
The opening of the Archenteron is called the
BLASTOPORE
Remnant of Blastocoel

27. 3: Gastrulation
3) The resulting Gastrula has two embryonic (or, germ) layers:
The ECTODERM on the outside,
and
The ENDODERM on the inside.

28. 3: Gastrulation
3) The resulting Gastrula has two embryonic (or, germ) layers:
The ECTODERM on the outside,
and
The ENDODERM on the inside.
The Mesoderm will form later, between the Ecto- and Endo-derm layers.

29. 3: Gastrulation BLASTOPORE ARCHENTERON
4) The Archenteron will eventually become the digestive tract and the blastopore either develops into the mouth or the anus.
In the case of Deuterosomes, like the Sea Urchin, the Starfish and humans, it will become the anus.
ARCHENTERON
BLASTOPORE

30. Development in a Starfish
You will use slides to examine the stages of development in a Starfish from Fertilization to Gastrula

31. 4: Neurulation
Neurulation only occurs in Chordates (all vertebrates and some closely related invertebrates)
Forms the brain and nerve (spinal) cord

32. 4: Neurulation The Process begins with the Ectoderm.
Certain cells of the Ectoderm flatten into a long NEURAL PLATE
ECTODERM
NEURAL PLATE

33. 4: Neurulation
The Neural Plate sinks and the edges of the ectoderm rise forming the Neural Groove and Neural Folds
NEURAL GROOVE
NEURAL FOLDS

34. 4: Neurulation
The Neural Folds approach each other, and fuse

35. 4: Neurulation NEURAL TUBE
The sides of the Neural Plate fuse and become the Neural Tube. The anterior end of this becomes the Brain and the posterior end becomes the spinal cord.
NEURAL TUBE

36. Development in a Frog
You will use slides to examine the stages of development in a Frog from Cleavage to Neurulation
Note the differences in blastulation between the isolethical and telolethical eggs

37. Development in the Zebrafish
The Zebrafish has a very telolecithal egg
Note how the Blastodisc forms like a cap on the surface of the yolk.
Gastrulation is different from the frog. The blastula surface slides down and engulfs the yolk, which ends up inside the embryo
In the live fish, observe the Somites, Nervous system (notochord and developing brain) and the circulatory system (beating heart and circulating blood)