The pre-embryonic stage is the first two weeks of development after fertilisation and involves three major processes: a) cleavage, b) implantation, c) embryogenesis.
a) Cleavage refers to the rapid mitotic divisions within the first 3 days. After fertilisation of the egg, it’s known as a zygote and the divisions within the zygote are very rapid, producing up to 100 cells but there is no significant growth.
Cleavage can be holoblastic (full) or meroblastic (partial) and depends on how much yolk is present within the egg. Humans and other mammals have little yolk as they receive nourishment from the mother (placenta or milk) during the embryonic stages and therefore undergo holoblastc cleavage.
Each cell produced is called a blastomere and the collective term for over 100 blastomeres is a blastula.
The blastula then forms a blastocyst, characterised by it's inner cell mass (pale blue lump on the inside of the cell differentiation image).
b) Within the blastocyst there’s an outer layer of simple squamous cells called the trophoblast that becomes the placenta and there’s an inner cell mass or embryoblast that’s destined to become the embryo itself.
When the blastocyst attaches to the endometrium, this is the beginning of implantation.
c) During implantation, the embryoblast undergoes gastrulation, which is the arrangement of the three germ layers: endoderm, mesoderm and ectoderm. Cells migrating inwards into the blastocyst form these.
After gastrulation, a process called organogenesis occurs. This starts with neurulation, when the neural plate (ectoderm) forms the neural tube, precursor of the central nervous system.
These germs layers will continue to form the digestive and respiratory tracts, muscle related tissues and nervous system respectively.
Implantation occurs in three stages:
Cleavage image courtesy of Gray's Anatomy under public domain: commons.wikimedia.org/wiki/File:Gray9.png
Cell Differentiation image courtesy of NCBI under public domain: commons.wikimedia.org/wiki/File:Cell_differentiation.gif