for class XII biology students

Development of frog

Embryology is the science which deals with early developmental stages of organisms. Father of modern embryology is Von Baer. Embryogeny is the process of development of embryo. Embryo is a newly forming individual in early stages of development before hatching or birth.
Embryonic development in all sexually reproducing metazoans begins from a single cell called zygote and follows similar sequence. The sequence of evnts before and during the embryonic development of frog includes.
Gametogenesis – formation and differentiation of sex cells or gametes.
Fertilization – fusion of male and female gametes to form zygote.
Blastulation – formation of segmentation cavity called Blastula.
Gastrulation – movement and arrangement of embryonic cells to form primary layers.
Organogenesis or organ formation – differentiation and formation of organ system from primary germ layers.
Differentiation and morphogenesis – acquisition of characteristic morphological features of organism.
Growth – increase in size through development.
Gametogenesis takes place little before the commencement of breeding season. The gametogenesis in male frog is called spermatogenesis in which sperms are developed inside the testis. Similarly gametogenesis in female frog is called oogenesis in which eggs are developed in side the ovary. During the rainy season which is the breeding season, male frogs croak and large number of frogs accumulated near the shallow water. In the water, male frog mounts upon the female frog and copulation takes place. At the time of copulation, female frog discharges number of eggs and near eggs male also discharges even large number of sperms. Along with eggs, wall of ovisac secretes albuminous fluid. The albuminous fluid absorbs water and swells up. Then it becomes sticky. It binds large number about 40 to 50 eggs in a group. This group of eggs is called frog spawn. The jelly like substance is formed from the albuminous fluid. It is mainly protective in function.
Functions of jelly like substance
Bacteria and fungi spores can not grow on jelly. So eggs are protected from bacteria and fungi.
Jelly is distasteful. Birds also do not feed on distasteful eggs.
Due to swelling of jelly eggs are moved apart and eggs are protected from friction.
Algae can grow in the space which can provide oxygen by photosynthesis.
Jelly can act as peculiar condenser which can absorb heat from sunlight. The temperature is increased. Higher temperature is needed for development of egg.
Structure of egg
The egg of frog is about 1.8mm in diameter and oval in shape. It is covered by double layered vitelline membrane. The egg is half black(pigmented) and half white. The black half is known as animal pole and white half is known as vegetal pole. The animal pole or hemisphere contains cytoplasm and nucleus and vegetal pole or hemisphere contains yolk. Yolk is found only in the vegetal pole, so it is called telolecithal egg or polar egg. There is moderate amount of yolk, so the egg of frog is called mesolecithal egg. In the freshly laid egg, under the vitelline membrane there is presence of the first polar body.
Structure of sperm
Sperm of has head, middle piece and a long vibratile tail. In the head, there is large nucleus and at the tip, there is acrosome with sperm lysine. It swims in water to reach to the egg of its kind. It is always attracted toward the egg of frog.
Process of fertilization
Fertilization takes place in water out side the body of frog. It is called the external fertilization. Large number of sperms remain attached to one egg. All sperms break the tip of the sperm. They release the sperm lysine. The change in the sperm to release the sperm lysine is called capacitaton. These changes take place due to the activation of sperms. Sperm lysine contains the enzyme hyaluronidase. It dissolves the vitelline membrane of egg. The egg also produces a small outgrowth towards the animal pole. It is called the reception cone. With this change, the egg is also activated to take part in the fertilization.
Now, one of the sperms attached to the reception cone pierces the vitelline membrane of egg and enters into it. The tail is left behind. Through the puncture, water oozes out and fills the space in between vitelline membrane and plasma membrane. It becomes thick and hard. It is known as fertilization membrane. Its function is to prevent the entry of other sperms. As the sperm head moves, it carries the pigment granules. It is marked by dark streak and known as the penetration path. The route followed by make nucleus in side the egg is copulation path.
The sperm nucleus is now known as male pronucleus. The egg nucleus undergoes the second meiotic division. As a result of this, second polar body is extruded. The egg completes the meiotic division only after the entry of sperm. The egg nucleus moves toward the male pronucleus. Just behind the male pronucleus , there is the centrosome. Due to the approaching egg nucleus, the male pronucleus rotates so that the centrosome lies in the middle of male pronucleus and egg nucleus. The nuclear membrane of both nuclei disappear. The chromosomes are set free. Spindle fibres are formed between the chromosomes. One chromosome from the male pronucleus and one similar chromosome from egg nucleus come closer and make homologous pair. Similarly several pairs of homologous chromosomes are formed. This restoration of diploid number of chromosomes is called the fertilization. At the end of fertilization, single celled zygote is formed.
Effects of fertilization
o The fertilized egg can undergo further development. If there is no fertilization, the egg will be degenerated.
o When the vitelline membrane is punctured, some water oozes out and collects in the space between vitelline membrane and plasma membrane. Now the vitelline membrane is changed into perivitelline membrane or fertilization membrane. It is stiff and thick. This ensures the monospermy of egg by preventing entry of other sperms.
o The second polar body is formed which shows that the egg has completed the meiotic division.
o Diploid number of chromosomes is restored.
o Centrosome is introduced.
o Fertilized egg rotates so that the animal pole turns upward and vegetal pole down ward. The significance of this is that the black surface can absorb heat from the sun light and increase the temperature. Higher temperature is needed for the development.
o Grey crescent is formed opposite to the point of entry of sperm. This is due to the movement of pigment granules. This shows the bilateral symmetry in the future.
Cleavage
After about three hours of fertilization, the fertilized egg starts to divide. The rapid and repeated divisions of fertilized egg is called cleavage. All these divisions are mitotic. During the cleavage, all substances present in the egg are divided. There is complete division. So, the cleavage in frog is holoblastic.
First cleavage
It is vertical division from animal pole to vegetal pole. A small depression appears at the animal pole. It becomes deeper and deeper and goes toward vegetal. The fertilized egg is divided into two equal cells. So, the first cleavage is vertical and equal.
Second cleavage
This is also vertical and equal but at right angle to the first. Two cells are divided into four quarter cells. Each cell contains a small portion of animal pole and a small portion of vegetal pole. These cells can be called as blastomeres. Both of these divisions are along meridian also.
Third cleavage
This division is horizontal and unequal. It takes place little above the equator and toward the animal pole. The four cells are divided into eight cells. The upper four cells are smaller and also known as micromeres or epiblasts. The lower four cells are bigger and also known as macromeres or megameres or hypoblasts.

Differences between micromeres and megameres
micromeres
smaller
Contain cytoplasm and nucleus
Black in colour, pigmented
Upper cells
Divide rapidly
megamere
bigger
contain yolk
white in colour, non pigmented
lower cells
Divide slowly because of the presence of non living, inactive yolk

Fourth cleavage
It is again vertical. Two furrows appear at the same time dividing all eight cells into 16 cells.
Fifth cleavage
It is also horizontal. In this case also two furrows appear at the same time, one above and one below the previous division. All sixteen cells are divided into thirty two cells. The sixth division onward, all divisions are haphazard.
Morula stage
Now the embryo is mulberry shaped with number of smaller cells. The animal pole contains many smaller black cells and the vegetal pole contains many bigger white cells. This stage is very short

Blastulation
The macromeres due to the presence of inactive yolk, divide slowly where as micromeres divide rapidly. So, the lines of division do not coincide. A gap is appeared between these two types of cells. This gap is inside the embryo and in between the animal pole and vegetal pole. It is known as segmentation cavity or incipient blastocoel. Bastocoel increases in size and is filled fluid. This stage is known as blastula. The roof of blastocoel is made by smaller, black cells and floor is made by bigger white cells. It is comparable to one layered stage.
At the blastula stage, some potential differentiation of cells sets in. the smaller pigmented cells of animal pole represent the prospective ectoderm. It further divides into prospective epidermis and prospective neural plate. Near the vegetal pole, there is prospective notochord. On either side of which there are two lateral plates(mesoderm). The rest of the cells of vegetal pole represent the prospective endoderm.

Gastrulation
It is the process of rearrangement prospective areas so that these different areas occupy definite position and proportion. Gastrulation in frog is completed in three steps.
Epiboly
The micromeres can divide rapidly. These cells migrate and spread over those of vegetal pole. The migration of these cells may be termed as formative movement. These cells cover the megamere leaving a small opening known as blastopore. At this time also these cells are dividing rapidly. These cell now move in between outer smaller cells and inner bigger cells. These cell make the chordamesoderm . this process of moving in between outer and inner cells is involution.
Intucking (invagination) or imboly
This is in pushing of bigger cells from blastopore. A groove is formed by this invagination. This groove is the beginning of second cavity archenteron. The margin above blastopore is dorsal lip. On the sides of blastopore there are lateral lips. As there is more invagination, the lips of blastopore becomes wide. The archenteron increases in size and blastocoels becomes reduced and crescentic in shape due to more in pushing some other yolk laden cells protrude out through the blastopore.
Contraction of lips of blastopore
As yolk laden cells protrude out through the blastopore, the blastopore becomes narrow. This is known as contraction of lips of blastopore. These cells make yolk plug.
The second cavity archenteron is formed. So the center of gravity is shifted. Due to this shifting, the embryo rotates now. The epidermis comes to anterior the neural plate to dorsal and he endoderm to posterior.
The gastrula stage is comparable to two layered stage. The outer layer is ectoderm and inner layer is chrodamesoderm + endoderm. The chordamesoderm and endoderm are not distinguishable. The roof of the archenteron is made by chorda mesoderm and floor by endodermal cells. These three steps occur simultaneously.
Significance of gastrulation
o Formation of different layers like ectoderm, chordamssoderm and endoderm
o Formation of different structures and organ these layers.
Neurulation

Due to rotation of embryo, the neural plate comes to lie at the dorsal surface. The neural plate becomes stiff and hard a little bit. It sinks down in the middle. The edges of neural plate come up. These edges are now known as neural folds. The depression in the middle forms the neural groove. These neural folds increase in height. They come closer to each other. These folds meet and fuse in the middle. The fusion proceeds anteriorly and posteriorly. Anteriorly it remains open by neuropore. Posteriorly it communicates with archenteron by neurenteric canal. Later on both of thwse are closed. A completely closed neural tube is formed having a swollen portion anteriorly. This stage is now known as neurula. It is little bit elongated and oval in shape.

During neurulation, some important changes occur at the roof of archenteron. Just below the place where neural tube is being formed, some cells of chordamesoderm separate,. He become vacuolated and become hard and stiff. These cells make a rod like structure. It is known as notochord.
The process of notochord formation form chord mesoderm is known notogenesis. The main function of notochord is to support the neural tube. Later on the swollen part of neural tube forms the different parts of brain. And tubular part forms the spinal cord. The notochord is replaced by backbone or vertebral column.

Coelom formation

The rest of cells of chordamesoderm forms two lateral plates. Both of these lateral plates split into outer somatic or parietal layer and inner visceral or splanchnic layer. In between outer and inner layer, these develop a gap called splanchnocoel. Later splanchnocoel is filled with fluid and gives rise to the formation of coelom. The fluid inside is called coelomic fluid. The coelom this formed is said to be schizoclelic coelom.
By this time, the chordamesoderm has been separated from the endoderm. The roof of archenteron is formed by endoderm. Now the embryo has three primary germinal layers namely ectoderm mesoderm and endoderm.



Fate of three primary germinal layers
All three layers are primary means they are equally important. They are germinal because they can be changed from one form to another form. The process of formation of different organs from either ectoderm, mesoderm or endoderm is called organogenesis.

Ectoderm
Epidermis of skin,
Epithelia lining of olfactory and auditory organs,
Lining of stomodaeum and proctodaeum,
Brain and spinal cord (CNS),
PNS and ANS,
Pituitary gland,
Sclera and retina of eye,
Enamel of tooth etc

Mesoderm
Dermis of skin,
Bones and cartilage,
Muscles ,
Circulatory system, heart, vessels, blood, lymph,
Spleen,
Excretory system, kidney,
Reproductive system, testis, ovary ,
Choroid of eye etc


Endoderm
Alimentary canal, associated glands liver and pancreas,
Respiratory tract, larynx, trachea, bronchioles, alveoli,
Lungs,
thyroid and parathyroid glands,
urinary bladder etc