Principle of Early Development - Vertebrates

There is general pattern of development in all vertebrates. The process can be divided into four stages: (1) Cleavage (2) Gastrulation (3) Organogenesis (4) Growth.

Cleavage

After a sperm penetrates an egg, the egg quickly surrounds itself with an enveloping coat, the fertilization membrane, and a series of cell division called cleavage begins.

A cycle of repeated mitotic division continues until a spherical mass of cells known as morula is formed. The cells do not grow between the cell divisions. As cleavage or division continues, cells begin to move apart, so that spaces appear among the cells in the centre of the mass. Cells keep pulling away from the central area, forming a fluid filled cavity known a blastocoel. This hollow-sphere embryo which develops at the end of cleavage is called a blastula. The cells of the morula and blastula are called blastomeres.

Gastrulation

Cell division continues but the whole embryo remains in same size at this early stage, the cells are becoming smaller as cleavage progresses.

Then the embryo enters a phase called gastrulation. During this phase there is rearrangement of cells. The essential feature is the formation of three layers of cells, with each layer capable of developing into special tissue.

Once gastrulation has occurred, the whole body has an outer coating the ectoderm (ecto; outside), and an inner lining the endoderm (endo; inside). Between the ectoderm and endoderm, new cells, proliferate, filling the part between the two original layers and eventually building the bulk of the embryo. This is the mesoderm. Ectoderm, endoderm and mesoderm are called germinal layers.

 Organogenesis

After formation of the three germinal layers, different cells of these layers start differentiating to form different organ rudiments. This phase constitute the beginning of organogenesis. The three germinal layers give rise to the structures as shown in the table below.

Organogenesis

Growth

It is a long phase. All the basic organ rudiments increase in size and also undergo many physiological changes. As a result fully functional organism is ready to start life on its own, then the embryo either hatches or is born.

Example: Hatching in birds and reptiles, birth in eutherian mammals (a mammal whose young develop within the womb surrounded by a placenta. Subclass: Eutheria).