4  Embryology of  Poultry

4.1  Blastodisc

The main feature is there is very little space available inside the shell of the egg. Thus, development starts as a flat disc on top of the yolk.

4.2 Gastrulation

4.3 Induction

Much of our knowledge of animal development comes from the study of the readily accessible embryos of lower vertebrates. It has been known for many years that the zygote of the frog exhibits a grey crescent caused by exposure of the lower layers of cytoplasm when the surface layers are dragged inwards by penetration of a spermatozoon. The grey crescent later becomes the dorsal lip of the blastopore - the channel that opens into the central cavity or archenteron of the gastrula. The dorsal lip of the blastopore eventually forms part of the inside layer of the gastrula (the archenteron roof) and later becomes the notochord. The prospective notochord then induces the formation of the nerve cord.  Many different parts of the body develop by induction. Embryonic induction may be caused by ribonucleoproteins.

4.4 Competence

The tissue responding to induction (the ectoderm in the case of neural tube formation) must exhibit competence.  Inappropriate tissues lacking competence do not normally exhibit a response to inducers. Thus, the prospective notochord cannot induce the formation of a nerve cord in an inappropriate layer of cells such as mesoderm. Only ectodermal cells exhibit competence for the formation of the nerve cord.

4.5 Chick at 24 hours

Although the chick is developing from anterior to posterior (from top to bottom of the micrograph below), you can see the primitive streak is posterior. Hensen's node is the edge where the cells are rolling downwards and anteriorly. The area opaca is the general mottled area all around the developing embryo where it is resting on the yolk, whereas the area pellucida is where the embryo is lifted off the yolk above the subgerminal cavity. Neural folds are visible where the ectoderm is starting to form the neural tube (leaving an anterior open end at the neuropore). On each side of the neural tube are blocks of somitic mesoderm. The notochord is faintly visible (it is ventral to the developing neural tube).

The neural tube develops as a roll of ectoderm sinking downwards into the embryo.

4.6 Chick at 33 hours

The three main parts of the brain are starting to swell up (prosencephalon, mesencephalon and rhombencephalon). The optic vesicle will become the eye. The heart seems strangely anterior, but this is because it is still developing from a tubular structure. The vitelline vein is starting to invade the yolk to bring in nutrients for the expanding embryo. The process whereby somites develop from clumped parts of the paraxial mesoderm is visible.

4.7 Chick at 56 hours

Torsion has occurred so the chick's head is resting sideways on the yolk instead of face-down into the yolk.  The two main parts of the rhombencephalon (metencephalon and myelencephalon) have developed.

Further information

Libraries contain many excellent embryology texts - so grab the first one you find.

S.F. Gilbert & A.M. Raunio (1997)  Embryology.  Constructing the Organism. Sinauer Associates, Sunderland, MA.
This is an amazing book covering the development of many phyla of animals. The birds and mammals each have their own chapter - thus, giving the main features in the fewest words.


The water colour diagrams originate from my own student notes which were made largely from ,
B.I. Balinsky. (1965). An Introduction to Embryology.  W.B. Saunders, Philadelphia.
The micrographs were made from whole mounts of chick embryos stained with carmine (which is why they are red).