Introduction

The cycle of life is a vital aspect of the growth and development of animals, fungi, and plants. It is a sequential process where the genotype of an embryo is translated into phenotype, thus leading to the formation of an adult organism. Developmental biology is the study of the building of organisms and the process of their development. This essay provides an overview of the cycle of life in animals.

Stages of Development

The stages of development in animals include fertilization, cleavage, gastrulation, organogenesis, birth, metamorphosis, and gametogenesis. The period from fertilization to hatching or birth is called embryogenesis.

Fertilization

Fertilization is the fusion of the male and female gamete cells (sperm and egg) which results in the initiation of development and the formation of a new individual. The fusion of the gamete nuclei results in the formation of the embryo genome, which helps guide the development of the embryo.

Cleavage

Cleavage is a series of rapid mitotic divisions that follow fertilization. This process divides the large volume of zygote cytoplasm into smaller cells called blastomeres. By the end of cleavage, the blastomeres form a sphere known as a blastula.

Gastrulation

The rate of mitotic division slows down after cleavage, and the blastomeres undergo significant rearrangements which result in the formation of three germ layers (endoderm, ectoderm, and mesoderm). This stage is known as gastrulation.

Organogenesis

The germ layers interact and rearrange themselves to form tissues and organs, a process known as organogenesis. Chemical signals are exchanged between the cells, leading to the formation of specific organs at specific sites. Some cells undergo long migrations from their origin to their final location. These migrating cells include blood cells, lymph cells, pigment cells, and gametes.

Metamorphosis

In many species, the organism that hatches from the egg or is born into the world is not sexually mature. It must undergo metamorphosis to become a sexually mature adult. The young organism is often referred to as a larva and may look significantly different from the adult. The larval stage is the longest in many species and is used for feeding or dispersal.

Gametogenesis

In many species, a group of cells is set aside to produce the next generation. These cells are the precursors of the gametes and are collectively referred to as germ cells. The germ cells eventually migrate to the gonads and differentiate into gametes. The development of gametes, called gametogenesis, is usually not completed until the organism is physically mature. At maturity, the gametes may be released and participate in fertilization to start a new embryo.

Conclusion

The cycle of life is a continuous process where an organism passes through various stages of development, leading to the formation of a mature adult that eventually undergoes senescence and dies. The nutrients of the dead organism often support the early embryogenesis of its offspring, thus renewing the cycle of life.