Animal Development
1. AD starts with the production and fusion of gametes and continues with the development of a multicellular embryo, the
        emergence of larvae and juvenile stages, growth and maturation to sexual maturity and the process of aging and then
        death.
2. Differentiation – where cells, tissues, and organs become specialized for a particular function.
3. Morphogenesis – the development of animals shape (body form)and organization
        a. mostly descriptive
        b. last 50 years, AD has become more amore concentrated on the genetics in control and regulation of development
4. Gametogenesis – the production of the gametes
5. Fertilization – the union of male and female gametes.
        a. each are called pronuclei before fusion.
6. Cleavage and Blastulation – the production of the multicellular blastula
7. Gastrulation – the formation of the three primary germ layers – ecto-, meso-, and endoderm.
8. Neurulation – the formation of the nervous system in chordates
9. Organogenesis – the development of organs from the three primary germ layers

I. STAGE 1 Preparation of the egg, fertilization, and cleavage
1. Development begins as egg and sperm prepare for fertilization.
2. Sperm develops flagellum, which will move the haploid genetic complement of the paternal parent to the egg, which
        contains the haploid complement of the mother.
3. The egg builds up food reserves called a yolk which is comprised of proteins and fats and used as food by early embryo
4. The egg can be of two types depending on amount and distribution of the yolk part of the egg.
        a. Isolecithal Eggs – have an evenly distributed amount of yolk through the unfertilized egg
        b. Telolecithal Eggs – have a large amount of yolk concentrated at one end.
                1. Two subtypes of Telolecithal Eggs
                        a. Moderately telolecithal
                        b. Strongly telolecithal
5. In moderately telolecithal the preponderance of yolk as it one end – called the vegetal hemisphere or pole.  The opposite
        end is called the animal pole.
6. In Strongly telolecithal eggs, the yolk is so massive that only a small cap of cells at the top undergoes division –
        looks like  a small disc on a big yellow ball
7. When egg and sperm meet up they fuse together their haploid genetic complements to form a diploid cell called a zygote
8. Mitosis – not meiosis – mitosis cell divisions of cleavage rapidly convert the zygote into a multicellular ball
        called a morula
9. The morula quickly goes through further divisions to become a hollow ball called a blastula
10. The cells of the blastula are called blastomeres
11. The blastocoel forms within the ball of cells
        a. Isolecithal eggs – blastocoel is centrally located
        b. Telolecithal eggs – blastocoel is found at the dividing cells in the animal pole.
12. Cleavage – type of cleavage depends on where the blastocoel is located (i.e., either in center or at end)
13. The tow types of cleavage patterns are:
        a. Holoblastic Cleavage – when divisions pass through entire or whole of cell.
        b. Meroblastic Cleavage – when blastoderm is in animal hemisphere/pole.
14. In isolecithal eggs the cleavage is holoblastic and blastocoel is centrally located because the impact on the yolk is
        minimal.
15. In moderately telolecithal eggs the cleavage is holoblastic as well and the blastocoel develops in the animal
        hemisphere/pole. In this case the yolk will impede cytoplasmic divisions and impact the size of the cells. However, if
        the entire egg is cleaved, cleavage is still considered holoblastic.  In this case the blastocoel develops in the animal
        pole and cells in this area will thus have less yolk that those cells in he vegetal pole.
16. In strongly telolecithal eggs cleavage type is meroblastic. Only the active cytoplasm divides produces a cap of cells on
        a massive yolk sac.  Cells are called a blastoderm, which is a small cap of cells at one end (the animal pole).  The
        blastocoel forms within the blastoderm.  In meroblastic cleavage the blastocoel forms between two layers of cells
        within the blastoderm.
17. Take home point - although the end result of cleavage, the formation of the blastula, is the same in ALL organisms the
        pattern of cleavage may differ.

II. STAGE 2 Gastrulation
1. Gastrulation transforms the hollow ball of cells (in holoblastic cleavage) or the cap of cells (in meroblastic cleavage)
        into a gastrula, which will have all 3 germ layers
2. Whereas cleavage is characterized by cell divisions, gastrulation id characterized by cell movement.
3. Surface cells migrate to the interior of the embryo in a process of invagination
4. The invaginated cells will form a new internal cavity called the archenteron.
5. The archenteron is lined with endodermal cells that will form the gut lining of the digestive tract.
6. The archenteron opens to the outside through the blastopore, which in deuterostomes becomes the anus.
7. In protostomes, the blastopore becomes the mouth.
8. The cells that remain on the surface of the embryo becomes the ectoderm
9. The third layer, the mesoderm, develops between the ectoderm and endoderm.

III. STAGE 3 Neurulation
1. Late in gastrulation, neurulation, the formation of the dorsal hollow nerve cord, begins to form.
2. This is strictly a Phylum Chordata process.
3. Certain dermal cells flatten into elongated neural plate extending from the dorsal end of the anterior end of the embryo.
4. The center of this plate sinks into the neural groove.
5. The edges of the plate become elevated to form neural folds which approach each other, touch, and eventually fuse.
6. This forms the neural tube.
7. The anterior end of the tube forms the brain while the posterior end forms the spinal chord.

IV. STAGE 4 Organogenesis
1. After the dermal layers and nervous system have been established, organogenesis, the formation of rudimentary organs
        and organ systems takes place.
2. Ectoderm, the source of the neural tube in Chordates also form the skin and associated glands
3. Mesoderm – goes on to form the somites and notochord in chordates
        a. Somites – later form the muscles, skeleton, gonads, excretory system, and circulatory system.
4.  In Non-chordates – lack somites and notochord – mesoderm goes on to produce the excretory, circulatory,
        and reproductive system.
5. Endoderm – develops into the lining of the digestive tract and associated organs such as liver, pancreas, and
    paradoxically, the lungs.