Periods of development

In higher, sexually reproducing animals, the developmental events can be put into three distinct periods: pre-embryonic (progenesis), embryonic and post-embryonic periods.

1. Pre-embryonic period involves gametogenesis. It is formation and the differentiation of male and female gametes (sperm and ova) in the testes and ovaries. It is pre-zygote period, and it important because contents of normal or mutant genes in their combination determine the quality of offspring.

2. Embryonic period. All the structural and physiological changes that a zygote undergoes till the formation of an adult form are called as embryonic changes and the process as embryonic development. This period includes the events from fertilization of the egg to the time of birth.

Embryonic development takes place in a sequence.

1) Fertilization – fusion of male and female gametes to give rise to a diploid zygote.

2) Cleavage – division of zygote.

3) Blastulation – formation of hollow ball of cells blastula.

4) Gastrulation – movement and rearrangement of cells of an embryo to form three primary germinal layers: ectoderm, endoderm, mesoderm.

5) Histo- and organogenesis or Organ formation . differentiation and development of organ systems from the primory germinal layers.

6) Differentiation and morphogenesis.

3. Post-embryonic period. It is period of development which extends from birth to death. It involves growth, adult hood, ageing and death.

4. Embryonic development.

Fertilization is a biological process which includes an orderly series of physico-chemical events. In human being the fertilization is internal and occurs in the fallopian tube. During insemination millions of sperms, about 200 million are deposited in the vagina but only one out of it fertilizes the ovum. The single diploid cell formed is called as zygote. It is beginning cell of the new multicellular individual.

The human egg or ova at ovulation is at the secondary oocyte stage. The ova is surrounded by radiating rows of follicle cells called corona radiata. An ovum is said to have a polarity. The region where the polar bodes are released, is called as the animal pole and the region diametrically opposite is referred as the vegetal pole.

Types of eggs

A. Depending on the amount of yolk present, eggs are of four types:

1) Alecithal eggs. These are almost free of yolk (in human Lutheran mammals)

2) Microlecithal eggs. In such eggs very little amount of yolk is present: egg of Branchiostoma

3) Mesolecithal eggs. Such eggs contain moderate amount of yolk (in frog)

4) Macrolecithal eggs. Such eggs contain a large amount of yolk (reptiles, birds, egg – laying mammals)

B. On base of distribution of yolk in ooplasm, eggs are considered to be of four types:

1) Iso- or homolecithal eggs: in such eggs yolk is uniformly distributed all over in the ooplasm as in egg of Branchiostana (ланцетник)

2) Telolecithal eggs: in such eggs yolk is concentrated (moderate amount of yolk) in the vegetal pole (in egg of frog - amphibians)

3) Centrolecithal eggs: yolk is localized in the center surrounded by a layer of cytoplasm. Such eggs are found in insects.

4) Meiolecithal eggs: in such eggs yolk occupies the entire ooplasm, only a small area for the nucleus as in eggs of reptiles, birds, egg-laying mammals

The basic pattern of sequential changes: zygote ^cleavage morula ^blastulation blastula ^gastulation gastrula ^{organogenesis} individual.

Cleavage. Immediately after fertilization the zygote undergoes the first mitotic division about 24 hours after fertilization the cleavage divisions which convert a large cytoplasmic mass into number of small cells clustered together forming a morula.

The pattern of cleavage may differ in different animals due to the varying amount of yolk present.

a) Holoblastic: In this case cleavage divides the zygote and the blastomeres. The human egg being alecithal (yolk-free) shows complete division or holoblastic cleavage.

b) Meroblastic cleavage: In takes place in the small, yolk-free and metabolically active part of the egg cytoplasm. It is found in telolecithal and mesolecithal eggs of reptiles, birds and the divisions are incomplete.

c) Discoidal (in birds) – is incomplete partial division of animal pole’s to form embryonic.

d) Superficial is incomplete cleavage of centrolecithal eggs (Arthropods)

Types of cleavage on the basis of its pattern it is of three types:

a) Radial cleavage: in this case the resultant blastomeres are symmetrically around the polar axis and are of the same size (in echinoderms, sponges, coelenterates)

b) Spiral cleavage: in this case the cleavage plane is oblique to the egg axis and blastomeres of adjoining tiers are alternately arranged (nematodes, annelids, molluscs)

c) Bilateral cleavage: in this case the mitotic spindles and cleavage planes remain bilaterally arranged to the egg axis (amphibians, cephalochordates)