Blastula differentiate into various regions and cells (blastomeres) of a particular region form a particular part of adult body, hence these areas are called presumptive areas and at this stage if cells of a particular area removed the adult lacks a particular part from its body.

In microlecithal eggs or alecithal eggs, the cleavage results in the production of closely packed blastomeres. The stage in which solid mass of blastomeres appears is said to be morula, whereas blastula is hollow with a cavity known as blastocoel.

In spite of their morphological, anatomical and physiological differences, the fundamental events in sexual reproduction are the same. These events can be categorized as follows:

The hypothalamus is a small region of the brain. It's located at the base of the brain, near the pituitary gland. It secretes hypothalamic hormone such as the GnRH or Gonadotropin stimulating hormone which further has effects on the pituitary gland. The acrosome is the anterior covering on the head of the sperm that consists of enzymes called lysins that can lyse the ovum membrane to penetrate and fertilize it. A Graafian follicle is a developed tertiary follicle and is responsible for the ovulation that occurs as a result of the surge in FSH and LH levels responsible for estrogen production. Leydig cells in the seminiferous tubules are responsible for the secretion of androgens such as testosterone. Parturition is the process of the delivery of the fetus and during this process, Relaxin is a hormone produced by the ovary and the placenta with important effects in the female reproductive system and during pregnancy. 

Parturition is the delivery or childbirth of baby from the uterus. Gestation is the period of development during the carrying of an embryo or fetus inside viviparous animals and the duration between pregnancy and birth. Ovulation is the process of release of the ovum from the Graafian follicle as it ruptures. Implantation is the process of attachment of the zygote to the endometrial lining of the uterus. Conception refers to the post-fertilization fusion of the egg and sperm into the zygote.

• Ageing is a complex process that varies as to how it affects different people and even different organs. Most gerontologists (people who study aging) feel that aging is due to the interaction of many lifelong influences. These influences include heredity, environment, culture, diet, exercise and leisure, past illnesses, and many other factors.
• Morphological changes during aging include the number of taste bud decreases, Decreased rate of cardiovascular functions and The reduction in the number of nephrons.
• So, the correct answer is 'All the above'.

According to immunity theory, the immune system is programmed to decline its functions over time, and this fact is the main cause of aging. Thymic involution (shrinking of the thymus with age) is believed to be the main age-related change in the immune system, and this alteration begins after puberty.

So, the correct option is ‘Degeneration of thymus’.

• Ageing is a complex process that varies as to how it affects different people and even different organs. Most gerontologists (people who study aging) feel that aging is due to the interaction of many lifelong influences. These influences include heredity, environment, culture, diet, exercise and leisure, past illnesses, and many other factors.
• Lipofuscin (age pigment) is a brown-yellow, electron-dense, autofluorescent material that accumulates progressively over time in lysosomes of postmitotic cells, such as neurons and cardiac myocytes. It causes ageing by the degradation of phospholipid of the plasma membrane.
  
• So, the correct answer is 'degradation of phospholipid of plasma membrane'.

• The thymus gland secretes a peptide hormone called Thymosin. Thymosin plays a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity. In addition, thymosins also promote the production of antibodies to provide humoral immunity.
• The thymus gland is degenerated in old individuals resulting in a decreased production of thymosins. As a result, the immune responses of old persons become weak.
• Hence Thymosin hormone controls the biological clock of ageing by promoting immunity in adult and by degeneration in older individuals.
  
• So, the correct answer is 'Thymosin'.

• Scab is a collection of dried blood cells and serum and sits on top of the skin surface.
• The outermost layer of our skin is composed of epithelial cells. As wounds heal, epithelial cells regenerate across the wound surface from the edges to close the wound. 
  
• The Epidermal growth factor induces the epithelial cells below the scab to undergo mitosis and facilitates closure of the wound.
• So, the correct answer is 'Epidermal growth factor'.

When ageing is progressing in the human body, different types of oxidising enzymes activity increases because of decreased rate of metabolism.

So, the correct option is ‘Malate dehydrogenase’.

A. Actin - plays an important role in embryogenesis, the healing of wounds and the invasivity of cancer cells.

B. Myosin – plays an important role in muscle contractions and  a variety of movements of nonmuscle cells, including cell division,

C. Collagen – plays a significant role in ageing.

D. Elastin - plays an important role in the biomechanical function of vital body tissues and organs such as the heart, lungs, skin and the vocal folds.

So, the correct option is ‘Collagen’.

A. Somatic mutation theory - posits that the accumulation of mutations in the genetic material of somatic cells as a function of time results in a decrease in cellular function.

B. Programmed senescence theory - refers to the idea that senescence in humans and other organisms is purposely caused by evolved biological mechanisms in order to obtain an evolutionary advantage.

C. wear and tear theory - asserts that the effects of ageing are caused by progressive damage to cells and body systems over time. Essentially, our bodies "wear out" due to use. Once they wear out, they can no longer function correctly.

D. Immunological theory - asserts that the process of human ageing is, in fact, a mild and generalized form of the prolonged auto-immune phenomenon.

So, the correct option is ‘Programmed senescence theory’.

Ageing starts with the disappearance of thymus according to immunity theory . Thymic involution (shrinking of the thymus with age) is believed to be the main age-related change in the immune system, and this alteration begins after puberty.

So, the correct option is ‘thymus gland’.

Increase in the age of the persons is called ageing. In this, loss or decrease of physical, physiological, behavioural activities takes place. The decrease in wear and tear, neurohormonal changes, the rate of metabolism, somatic mutations take place. Replacement of organs does not take place in ageing.

So, the correct option is ‘Epimorphosis’.

Immunity theory is the widely accepted theory for ageing. The immune system is programmed to decline its functions over time, and this fact is the main cause of aging. Thymic involution (shrinking of the thymus with age) is believed to be the main age-related change in the immune system, and this alteration begins after puberty.

So, the correct option is ‘Non functioning of thymus gland’.

A. Error and damage theories - emphasize environmental assaults to living organisms that induce cumulative damage at various levels as the cause of aging.

B. Hormonal theory - hormones or reduced production of hormones might cause aging has also led some to believe that they could be an anti-aging elixir. 

C. Immunological theory - asserts that the process of human aging is, in fact, a mild and generalized form of prolonged auto-immune phenomenon.

D. Programmed senescence theory - refers to the idea that senescence in humans and other organisms is purposely caused by evolved biological mechanisms in order to obtain an evolutionary advantage.

So, the correct option is ‘Error and damage theories’.