The Zygote
The zygote is totipotent - it has the potential to develop into any other cell
Zygotes from different species differ in their yolk content:
- oligolecithal eggs - little yolk
- mesolecithal eggs - moderate amounts of yolk
- macrolecithal eggs - large amounts of yolk
The distribution of yolk also differs:
- oligolecithal eggs tend to be isolecithal - yolk is distributed throughout the egg
- meso- and macrolecithal eggs tend to be telolecithal - yolk is segregated toward the vegetal pole, away from the animal pole.

Cleavage

Early cell division occurs without cell growth, and is termed cleavage
The presence of yolk slows cleavage in proportion to its concentration
- in holoblastic cleavage cell divisions are complete
- in meroblastic cleavage, the yolk is not completely divided
Cleavage results in the blastula - a mass of undifferentiated cells
- cells have a high nucleus:cytoplasm ratio
- cells are undifferentiated
- the blastula contains a central cavity or blastocoel


Gastrulation
Gastrulation is the transformation of the blastula into a gastrula, a structure with three germ layers and a gastrocoel
Gastrulation begins at the blastopore - a spot where proliferating cells fold into the blastocoel
The gastrula has several important characters
the three main body axes are defined
cells acquire developmental fates
- the ectoderm will form the epidermis, nervous system, and sense organs
- the endoderm will form the gut lining and derivatives
- the mesoderm will form everything else


Neurulation
Neurulation is the initial formation of the nervous system
- growth, cell differentiation and organogenesis begin
Chordamesoderm cells aggregate to form the notochord
The chordamesoderm induces the dorsal ectoderm to form the neural tube
- induction involves turning on or off of specific genetic pathways in one type of cell following contact by another type of cell
Mesoderm flanking the notochord becomes segmented into laterally paired blocks or somites
different regions of the somites and lateral mesoderm have different developmental fates
Pharyngeal pouches (6-9) develop by evaginations of the pharyngeal endoderm, which meet indentations of the ectoderm
- they give rise to several important structures:
(e.g., the eardrum, jaws, glands)
Placodes are ectodermal thickenings in the head
- they contribute to the sense organs, and to the wandering cells
Mesenchyme is the embryonic connective tissue
- Wandering cells are mesenchyme cells that migrate through the embryo and form specific structures
- Neural crest cells are wandering cells derived from ectoderm dorsal to the neural tube.
- unique to vertebrates


Organogenesis
Organogenesis involves continued specialization of cells to form tissues
- ectoderm and endoderm form predominantly epithelium - sheets with tight intercellular junctions, flanking open space
- mesoderm forms the matrix of the organs, including a variety of tissues
Tissues combine to form organs
Organs unite into organ systems