Unfertilized Eggs : A Living Entity
Unfertilized Eggs : Some people have the notion that unfertilized eggs , especially of hens, are inanimate and thus they can be part of a vegetarian diet. The only argument that they have is that no chicken may be born from an unfertilized egg. They ignore the signs and characteristics of life which are present in an unfertilized egg. They are also unaware of the fact that there is a potential capability in the unfertilized ovum (the mature female germ cell which develops into a new member of the same species) to bring out offspring through a process known as artificial parthenogenesis (development of an ovum stimulated by chemical or mechanical means).
Development of an egg:
Biologists have found more than a million different kinds of life on earth. Despite the great diversity of life, there is also a great unity of life. All living beings are alike in many ways. Though biologists have not been able to agree on a simple definition of life that fits all cases, they do agree on what the ‘signs of life’ are. Before tracing these signs of life in an unfertilized chicken egg, it is necessary to know the genesis (process of formation) of an egg.
Webster’s dictionary defines an egg as ‘the oval or round body laid by a female bird, fish, reptile, insect, etc., containing the germ of a new individual along with the food for its development, and having an enclosing shell or membrane’. It is also called an ovum, a female gamete or female reproductive cell or reproductive organism. In sexual reproduction, the ovum unites with its male counterpart which is called sperm cell. This fusion in which the first cell of the new organism is formed is called fertilization. The ovum contains half the chromosomes (microscopic bodies which carry the genes that convey hereditary characteristics) required by the new organism while the sperm supplies the other half.
An ovum is produced in the female reproductive gland, the ovary. “When liberated from the ovary, an ovum begins to undergo certain changes that can be characterized as aging or deterioration. Among other things, there is a tendency for its protoplasm (the essential living matter) to become progressively more coarse.”This causes the ovum to lose its vigor. “These changes progress rapidly to a point where the ovum, although technically still alive, can no longer be fertilized. If, however, the ovum is fertilized reasonably promptly, these deteriorative changes are checked and the protoplasm increases its activity in a way that is often described as ‘being rejuvenated’.” One aspect of the rejuvenation is an increase in the rate of oxidation and in the excretion of ammonia. These changes signify an increase in purine metabolism (chemical and physical processes which go on in living organisms). This marks the beginning of rapid growth ending in the production of a new individual.
Interestingly enough, fertilization in the usual manner by the male sperm cell is not the only way an ovum may have its deterioration checked and be started on its growth phase. A variety of other stimuli may be substituted for the male sex cell. Changes in the ionic concentration (number of charged particles) of sea water is effective in initiating development in the eggs of some marine invertebrates. The development of insect eggs can be started by stroking them with a camel’s hair brush. Pricking the eggs with a needle has been found to be effective in inducing growth of frog’s eggs. Such initiation of development in an egg by means other than its union with male sex cell is known as artificial parthenogenesis. The fact that artificial parthenogenesis is possible indicates that all the necessary morphogenetic factors for the development are present in the ovum and that the process of fertilization is essentially, as Barth puts it, a sort of ‘release mechanism’. In other words, it seems that the union with the sperm cell or the chemical or mechanical factors employed in artificial parthenogenesis release the potential energy already present in the egg. It should, however, be pointed out that the sperm brings with it the quota of hereditary potentialities (chromosomes).
An egg (ovum) begins its development in the ovary of a female. An ovum is a reproductive cell of the germinal epithelium (cellular tissue). It is contained in a very thin envelope called the follicle which is attached to the ovary by a slender stalk. Many ova are present in an ovary and grow in size as the bird (hen) comes into laying condition.
Yolk granules accumulate in layers within the ovum until the yolk of the egg becomes as large as that of a freshly-laid egg. At that stage of development, the follicle bursts and the yolk is freed from the ovary. The yolk is picked up by the mouth of the oviduct (a tube through which ova pass from the ovary to the uterus) where development of the inside of the egg is completed. First, the yolk becomes surrounded by the thick albumin. As the yolk travels through the oviduct, shell membranes are added. The rest of the formation of the egg occurs in the uterus where additional albumin is added and the shell is formed.
Hens managed for commercial egg production are kept in a specially controlled environment that provides 14 to 16 hours of artificial or natural light. The birds respond to a gradual increase in the amount of daylight. A tiny gland, the pituitary, located at the base of the brain, secrets hormones which speed up the development of the ova. When the exposure to light is increased, the activity of the pituitary gland goes up, more hormones are secreted. Thus the birds are stimulated to lay more eggs.
A bird’s egg has five principal parts: the shell, the shell membrane, the white, the yolk and the ‘germ’ (nucleus). The most important part of an egg is the nucleus which develops into the new animal. The other parts of the egg provide food and proteins for the young animal. The shell has tiny holes so water and gases can pass through it. The shell membrane is located just inside the shell. Water and gases can pass through the membrane. The white of an egg surrounds the yolk which in turn is contained in a membrane commonly known as yolk sack. When a fertile egg is kept under suitable conditions, the germ develops into an embryo. The contents of an egg make up a well-balanced diet for the developing embryo. For example, the egg shell, which is composed mostly of calcium carbonate, provides the embryo with calcium for the formation of bones and other body-building materials. A clear understanding of these facts indicates that even an unfertilized egg has the eight signs of life discussed in the following paragraphs.
Characteristics of life:
Each living thing is called an organism. All organisms have the following general characteristics:
1. Living things are highly organized and contain many complex substances.
An egg is a highly organized organism (cell) and contains the following chemical substances: water (65%), proteins (12%), fat (11%), carbohydrates (1%) and ash (11%). Most of these substances are complex in nature.
2. Living things contain one or more cells which are the smallest units that can be said to be alive.
All cells carry on their own life activities.
New cells can be produced from existing living cells by the process of cell division or reproduction.
The ovum and later on the completed egg both constitute one large cell. The development of the ovum into the completed egg constitutes its life activity. The unfertilized egg cell (just like the fertilized one) is derived from the living cells of the ovary by the process of meiosis (reduction division). As a result of this reduction division, the number of chromosomes which is about 38 in a normal somatic cell, becomes half (that is 19) in the ovum (the unfertilized egg). When this ovum is fertilized by spermatozoa (male germ cells), the number of chromosomes become 38 again.
An egg, fertilized or unfertilized, contains all cell parts, that is cell wall, cell membrane, nucleus and cytoplasm. Each part has its specific function. Like any other normal cell, an egg also contains organelles such as centriole, which are characterized by having specialized functions. The process of development of an unfertilized egg is similar to that of a fertilized egg.
3. Living things use energy.
It is obvious that an egg uses a significant amount of energy in its development from a very minute ovum to a complete egg having a surface area of approximately 68 square centimeters and a weight of about 58 grams.
4. Living things have a definite form and a limited size.
Egg has a definite shape. In addition to the weight and surface area given above, an egg has approximately a volume of 63 cubic centimeter, long circumference of 16 centimeter, short circumference of 14 centimeters, shape index of 74 and specific gravity of 1.09.
5. Living things have a limited life-span.
As soon as the egg is released from the ovary, its aging process begins. As mentioned above, there is a tendency for an ovum to lose its vigor and for its the protoplasm (the essential living matter) to become progressively more coarse. If these changes are not checked by fertilization, then the ovum dies. An egg also perishes on account of unsuitable changes in the temperature of its environment.
6. Living things grow.
As mentioned above, an egg grows from a tiny ovum to a size of 63 cubic centimeters.
7. Living things respond to changes in the environment.
An egg responds to variations in the temperature and other factors.
8. Living things reproduce.
Scientists have been successful in producing identical offspring by making use of the chromosomes of a somatic (bodily) cell without taking recourse to the other partner in any manner. The cells involved in this case could not have produced a living offspring unless they were themselves alive. This indicates that an unfertilized egg is alive although it has chromosomes derived only from a female.
Besides the above general characteristics, the living things, in order to stay alive, carry on certain activities or functions that are also characteristics of life. These life processes are nutrition, transport, respiration, synthesis and assimilation, growth, excretion, regulation and reproduction.Growth and reproduction have been discussed above. Let us examine whether an unfertilized egg performs the rest of the functions.
An egg takes material from its environment and changes it into usable forms. This activity is called nutrition. Substances that an organism can use for energy or for growth and repair are called ‘nutrients’. The primary ovum derives its nutrition through the blood vessels while it is attached and growing in the ovary. Further nutrition for energy and growth is derived from the oviduct and uterus. Proteins, fats and carbohydrates are manufactured.
Transport and Excretion
Transport is the process by which usable materials are taken into the organism (absorption) and distributed throughout the organism (circulation). Wastes and other products of life processes are also transported from one place to another within the organism. In the smallest organism, there is no real transport system. Usable materials are absorbed directly into the organism from the environment. Wastes pass from the organism directly into the environment. In most animals there is a highly specialized circulatory system for carrying needed materials to all parts of the organism and carrying waste products away.
In an egg the gases, for example, carbon dioxide, and other waste materials directly pass through the shell membranes and shell into the environment and air from the environment is taken inside through the shell and shell membranes.
All life processes require a constant supply of energy. Organisms obtain the needed energy by releasing chemical energy stored in the nutrients. The process by which this is accomplished is called ‘respiration’. Respiration involves a complex series of chemical reactions. In one type of respiration, sugar is broken down to produce water and carbon dioxide. This is called ‘aerobic’ respiration because it requires oxygen from air. Breathing is the process by which air is drawn into the body and waste gases are eliminated. As mentioned under ‘transport’, an unfertilized egg performs this function.
Synthesis and Elimination
Organisms are able to combine simple substances chemically to form more complex substances. This process is called synthesis. The substances used in synthesis are generally products of the digestion of complex food materials. One of the results of synthesis is to produce materials that can become part of the structure of an organism. In this way, the organism can grow and repair or replace worn out parts. The incorporation of materials into the body of the organism is called assimilation.
The egg performs the synthesis of proteins, fats and carbohydrates and eliminates carbon dioxide, ammonia and other waste products.
Unicellular organisms (those having a single cell) are able to carry on all life processes. They synthesize and obtain nutrients, break them down for energy and synthesize new materials. An egg, unfertilized or fertilized, performs these functions during growth.
Regulation is the process by which an organism maintains a stable internal environment in a constantly changing external environment. This process is called ‘homeostasis’ which checks deterioration and ultimately death of the living organism. Obviously, an egg performs this function and that is why it survives.
Many biologists consider metabolic activity to be the single most important characteristic of life. The chemical reactions involved in the life processes of an organism are called its ‘metabolism’. Metabolism includes processes that break down complex substances into simple ones and processes that build complex substances from simple ones. Metabolism also involves the continuous release and use of energy. It is evident from the above discussion that metabolic activity is present even in an unfertilized egg.
In sum, all scientific evidence establishes the fact that an unfertilized egg is a living entity.