Essential stages in animal development essay

After fertilization, cleavage occurs. Rapid mitosis division takes place where the zygote cytoplasm is divided into several smaller cells termed blastomeres.

The rate of the mitotic division gets slower and makes the blastomeres undergo a series of changes. The embryo develops in the gastrula stage. Thus, the embryo has three germ layers: ectoderm, endoderm, and mesoderm.

Once the germ layers are arranged, cells together form tissues and organs. This process is called organogenesis.

The specialized portion of the egg cytoplasm gives rise to cells that are the precursors of the gametes collectively called germ cells. Other cells are called somatic cells.

The secretion of somatic cells to give revolt to an independent body and germ cells (which contribute to the formation of the new generation) is considered to be the first differentiation that occurs during animal development.

The germ cells migrate to gonads and are later differentiated into gametes. The development of gametes is called gametogenesis.

In many animals, the larval stage lasts longer and the adult is a brief stage responsible for reproduction. In silkworm, the mouth is not completely developed and cannot eat. The larva has to feed enough for adults to survive.


The animal kingdom is classified into protozoa and metazoan. Unicellular organisms come under protozoa and multicellular organism comes under metazoan (also called as animals). Paramecium and Amoeba are examples of protozoa. Mammals and reptiles are examples of metazoan.

Metazoan belongs to one of the following branches:




Phylum Porifera:
Phylum Porifera (pore-bearing species) is considered to be the lowest multicellular organism that belongs to the animal kingdom. They possess a spongy appearance. They usually do not move their body but is fixed to the substratum. They possess the ability to absorb nutrients from the water surface through their sponge-like structure. Initially, it has been declared as a plant and later was included in the animal kingdom based on its characteristics. Parazoa is the sub-kingdom of phylum Porifera. Sponges come under parazoa rather than metazoa. Sycon, Hylonema, Cliona, Euplectella, Spongilla are some examples of parazoan.


Asymmetrical body

Cells are loosely packed

No organ is present in their body

Their body possesses a cylindrical shape

The body consists of several pores known as Ostia and Osculum

Reproduction occurs asexually by fragmentation and budding

They can regenerate its lost parts

Gaseous exchange and nitrogenous waste occur by the process of diffusion Phylum Porifera is further distinguished into three subclasses as follows:

Calcarea (Calcareous Sponges)

Hexactinellids (glass sponges)

Desmospongiae (demosponges)

Diploblastic animals possess a body wall composed of two layers; ectoderm and endoderm. Mesoderm is absent in diplomatic animals. Coelenterates (for instance Cnidarians and Ctenophores) are diploblastic. The diploblastic animals with radial symmetry are called Radiata.

Triploblastic animals possess three germ layers namely ectoderm, endoderm, and mesoderm. Based on the bilateral symmetrical and triploblastic characteristics is further classified into protostomes and deuterostomes. For example – all multicellular organisms except sponges and coelenterates.

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