1. Morphogenesis Vertebrate eggs Early chordate development
Fate of the germ layers
Extraembryonic membranes

2. Vertebrate Eggs Size and clevage pattern depends on amount of yolk present
Microlecithal - “little yolk”
Mesolecithal - “moderate” yolk
Macrolecithal - “large” yolk

3. Distribution of yolk:
Isolecithal - even distribution
Telolecithal - yolk is more concentrated in one hemisphere

4. Disposition of eggs:
Oviparity- eggs are shed into the environment
Viviparity- eggs are retained internally and living young delivered
- Ovoviviparous: young nourished by yolk
- Euviviparous: young nourished by maternal tissues (histotrophic or placental)

5. Early Embryo Development
Clevage
Refers to division of cells
Pattern determined by amount and distribution of yolk
(1) Holoblastic: entire egg cleaves (equal or unequal)
(2) Meroblastic: only a small disc of cytoplasm cleaves

6. Blastula
Hollow ball of cells (holoblastic) or a “cap” of blastoderm (meroblastic)
Each cell is a blastomere
Cavity is the blastocoele
Massive amounts of yolk result in formation of the blastoderm
Placental mammals produce a blastocyst consisting of trophoblast and inner cell mass

7. Gastrulation The most important event in your life
Dynamic process of cellular movements which form the three primordial germ layers and the coelom
Morphogenetic movements depend on the amount and distribution of yolk.
Involves some type of involution.
Regulated by homeobox genes and their proteins - morphogens

8. Primary Germ Layers Formed during gastrulation
Ectoderm: outerlayers of integument and nervous system (neural crest and neurectoderm)
Mesoderm: muscle, bone and connective tissue. Dorsal (somite) mesoderm, intermediate and lateral
Endoderm: lining of the gut (yolk sac endoderm - primordial germ cells

9. Extraembryonic Membranes
Vitelline: surrounds yolk sac; highly vascular and confluent with embryonic circulatory system
Amnion: encloses embryo to protect against dessication (water loss); terrestrial vertebrates
Chorion: surrounds amnion, contacts maternal tissue or egg shell
Allantois: evagination from cloaca; forms part of mammalian bladder.