Growth And Development Of Animal

We know that growth and development are interconnected. Growth is the irreversible addition of cells and body mass. Development is growth followed by organogenesis and differentiation of structures and functions. There are two stages of animal growth and development: embryonic and post-embryonic. 

Embryonic Stage 

The embryonic stage starts at fertilization (union of egg and sperm cells) that forms a zygote. The zygote then undergoes several cleavages. There are several phases of the embryonic stage: morula, blastula, gastrula, differentiation phase, and organogenesis. We will discuss each of these phases. 

Morula Phase 

At this phase, the zygote cleaves/ divides several times, from one into two, two to four, and so on. During cleavage, there is 

Blastula Phase 

During the blastula phase, cytoplasm is divided into two poles formed during morula phase. There is different amount of cytoplasm on these two poles. Cells of functional (animal) pole have less cytoplasm than those of vegetal pole. This difference determines the direction of growth and development of the animal. Functional and vegetal poles are ready and formed in this phase. It is marked by the formation of fluid-filled cavity between the two poles, called blastocoel (Figure 1.15). Embryo with blastocoels is called blastula. The process of forming blastocoels is blastulation. This phase is continued by gastrula phase. 

Gastrula Phase 

During gastrula phase, embryo undergoes a differentiation process by starting to eliminate the blastocoel. The Cells of the functional pole divide rapidly. As a result, cells of the vegetal pole form an inward curve (invagination). Invagination forms two types of strutures: outer layer (ectoderm) and inner layer (endoderm). 

Ectoderm Will form the skin. The endoderm will form many types of tracts and guts. The embryo at this phase is called the gastrula. The middle part of gastrula is called the archenteron. During the next developmental stage, the archenteron becomes the digestive tracts in vertebrates and some invertebrates. There is an opening on the gastrula that heads toward the archenteron, called a blastopore. This structure will make an anus and its other end will open and become the mouth. Some part of the endoderm continues differentiation into a mesoderm. During the end phase of gastrulation, the endoderm, mesoderm, and ectoderm are fully formed (Figure 1.16). 

Based on the number of embryonic layers, animals are grouped into two kinds: diploblastic and triploblastic animals. Diploblastic animals have two embryonic layers: ectoderm and endoderm. An example of a diploblastic animal is a coelenteratas. Triploblastic animals have three embryonic layers: ectoderm, endoderm, and mesoderm. The mesoderm is always located between the ectoderm and endoderm. 

Triploblastic animals are grouped into three subgroups based on the presence of the coelom (body cavity; originates from the word coelom=hollow space or cavity) and how it is formed during embryogenesis. These subgroups are acoelomata, pseudocoelomata, and coelomata (eucoelomata). Acoelomata lacks a coelom, for example flat worms (Platyhelminthes). Pseudocoelomata has a pseudo coelom, for example, earth worms. Coelomata organisms have a true coelom, such as humans (Figure 1.17). 

Differentiation and Organogenesis 

During this phase, differentiation and organogenesis occur in cellular structures and functions to become Specific tissues. This process is controlled by hereditary factors (genes) brought from the formation of functional and vegetal poles. Eventually, each part of the endoderm, mesoderm, and ectoderm undergoes differentiation into the following organs. 

* Ectoderm differentiates into epidermis, hair, oil glands, sweat glands, tooth enamel, nervous system, and receptor nerves. 

* Mesoderm differentiates into bones, connective tissues, muscles, circulatory system, excretory system such as the vas deferens, and reproductive system. 

* The endoderm differentiates into digestive epithelial tissues, respiratory system, pancreas, liver, and thyroid gland. 

During differentiation and organogenesis, structures that are in close proximity affect one another. For example, the mesoderm affects ectoderm in differentiation of movement organs. Some structures come from the ectoderm and some from the mesoderm. After the embryonic stage is passed, the embryo that is called a fetus is ready to be delivered. 

Post-Embryonic Stage 

At this post-embryonic stage, organisms undergoes growth and development to become adult individuals. The adult individual is ready to reproduce and produce offspring. Some invertebrates undergo regeneration or metamorphosis during their growth and development. Vertebrates undergo growth and development from young animals into mature adult animals. 

Regeneration 

Regeneration is the process of body repair or healing from wounds or damages. This process is determined by undifferentiated stem cells in the animal’s body. In organisms that reproduce asexually, regeneration 

can also serve as a reproductive process. An example is in flat worms. Flat worms have a high regenerative ability. When a_ flat worm’s body is cut into some pieces, each piece may 

develop into a new and complete individual (Figure 1.18). 

Metamorphosis 

Metamorphosis is the change of size, shape, and structures of animal’s body from one stage to another. Metamorphosis specifically is a growth and developmental process in maturing insects and amphibians. The animals undergo metamorphosis have different body structures and functions at different stages. Metamorphosis is controlled by hormones. Under hormonal influence, body size increases, tissues are differentiated, and body structures are formed. 

Metamorphosis of insects Based on the presence and the state of metamorphosis, there are ametabolous, holometabolous, and hemimetabolous insects. 

*Ametabolous insects 

Ametobolous insects do not undergo metamorphosis.Their1ife cycle starts from egg stage to imago (adult) stage. An example is the bristletail that develops into an adult form 

directly from an egg without metamorphosis ' (Figure 1.19).

° Holometabolous insects 

Holometabolous insects undergo complete metamorphosis. The life cycle is staged from an egg, larva, pupa, and imago (adult). An example of an insect that undergoes complete metamorphosis is a butterfly. We can observe its egg stage on a lime leaf. The egg hatches into a very active larva. It eats the lime leaves. The larva undergoes skin moulting several times called ecdysis. Afterward, the larva turns into a pupa. The pupa phase is a resting stage. The organism inside the pupa develops into a butterfly that can fly and produce eggs. The whole cycle repeats again (Figure 1.20). Other examples of holometabolous insects are beetles, moths, ants, and bees. 

Hemimetabolous insects 

Hemimetabola insects undergo an incomplete metamorphosis. The life cycle starts from egg, larva or nymph, semi~imago, and imago (adult). The examples are some beetles. We can observe the egg stage in the sand as it’s attachment medium. The eggs hatch to become larvae. The semi-imago stage will then form. This stage of insect has an identical morphology to imago beetle. However, it has not yet able to reproduce because its reproductive system has not yet developed properly. The beetle then enters the stage of image when it can reproduce and lay eggs (Figure 1.21). Other examples of hemimetabolous insects are locusts, cockroaches, and termite. 

Metamorphosis Offrog (amphibians) 

There are generally three stages Of frog metamorphosis: premetamorphosis, prometamorphosis, and climax metamorphosis. 

During premetamorphosis stage, fertilized eggs grow into tadpoles. Tadpoles increase in size With little change in body form. During prometamorphosis, hind limbs appear and the body grows slowly. During climax metamorphosis, front limbs appear and tail disappears. Look at Figure 1.22. 

Some morphological and physiological changes during frog metamorphosis can be read in Table 1.3. 

The changes in body shape of frog during metamorphosis are influenced by hormonal activity of thyroxine. Thyroxine is controlled by TSH (Thyroid Stimulating Hormone) produced by the hypophysis (pituitary gland). 

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