The first cleavage of a zygote occurs immediately after . Before the zygote is implanted into the uterine wall at day 7-10, it undergoes a series of subdivisions without . The cleavage generates a - a hollow ball of cells consisiting of an outer layer of cells known as the . This outer layer will interact with the wall. Inside the is the which will develop into the embryo.

Fill in the blanks below to describe the process of gastrulation.
1. The primitive streak develops in the .
2. The end of the primitive streak expands to form the .
3. Cells of the epiblast migrate inwards towards the streak, detach from their layer and migrate it.
4. The first cells of the epiblast to migrate invade the layer, displacing its cells.
5. Eventually, the hypoblast cells will be entirely replaced by a new layer. This layer is the definitive .
6. Some of the invaginated epiblast cells remain in the space between the layers. This layer is the .
7. The remaining cells of the epiblast form the definitive .

Using the image, fill in the blanks to describe each stage of neurulation.
1. Neuroectodermal cells differentiate to form the with a border to separate it from regular ectoderm.
2. The neural plate folds upwards with its apex extending . The two edges join at the , forming the .
3. The closure of the tube disconnects the neural crest from the wall. The cells of the neural crest then differentiate to form the .
4. The degenerates and other mesoderm cells differentiate into the .

Fill in the blanks to describe what each of the primitive tissues will develop into.
The mesoderm will form the splanchnic and somatic nerves.
The mesoderm will form the kidneys and the gonads.
The mesoderm will form the head and the somites.

Fill in the blanks to describe the process of neural crest migration.
The first wave of migration forms the ganglia. The parasympathetic ganglia migrate to the walls of the . The sympathetic ganglia migrate in front of the .
The second wave of migration forms the ganglia either side of the spinal cord. It also forms the cells and satellite cells.
The final wave of migration forms the throughout the epidermis. These give pigmentation to the hair, skin, iris etc.
The neural crest cells at the cranial end form the of the cranial nerves. They also migrate to form the of the face and neck.

A mutation in the gene can result in holoprosencephaly. This results in a lack of development of the two hemispheres of the . It often presents with microcephaly and/or a cleft palate/lip. is the most extreme form of this condition whereby the orbits of the eye do not divide into two cavities, leaving a single central eye.

The paraxial mesoderm forms the head and the somites.
The somites will then differentiate into...
The - this will form the vertebrae and the rib cartilage.
The - this will form the muscles.
The - this will form the skin and connective tissue.