The head of the embryo grows forward
And the gut elongates rearward
The neural folds meet in the brain region, but fuse
At this stage, four pairs of somites, blocks of mesodermal tissue, have formed
As the head of the embryo grows forward
The head fold further undercuts its anterior end
The neural folds fuse to form new sets
Anterior end develops into a three-part brain
Early structures of the eye are now prominent
Meanwhile
The embryo develops in an equally exciting manner
In this later stage
And the small embryo is already visible
As a light-colored disk
Soon, blood vessels appear outside the embryo
The heart develops and begins to beat
The pattern of blood vessels over the egg expands
Supplying the embryo with food and oxygen and removing waste
As the embryo grows, the brain, eyes, and other structures develop
Soon, a vigorously beating heart is pumping blood throughout the complex embryo
Which is rapidly developing into a mature organism
The blastoderm, the area pellucida, is translucent because it is over a liquid
And the white of the egg
Vessels over the yolk expands
Some of the albumen becomes thickened and twisted as the egg passes down the oviduct
These rope-like strands, called the chalazae, help to hold the cell in position
The blastoderm at this stage consists of many cells produced by repeated cell division
While the egg is still within the organism