Introduction:
- In sexual reproduction, development of organisms begins with a fertilized egg or zygote.
- Zygote divides and re-divides to form an embryo.
- Embryo is the earliest stage of development, while it is within the egg or reproductive organs of mother until hatching or birth.
- The branch of biology dealing with the study of embryo is known as embryology.
- The development of embryo takes place in an orderly sequence and follows the same fundamentally similar sequence of events in almost all sexually reproducing organisms.
You may also want to see: The process of fertilization and development of embryo in angiosperms
Sequence of events before and during embryonic development in living organisms:
- Gametogenesis: Formation of gametes or sex cells.
- Mating: Transfer of male gametes inside female’s body by copulation.
- Fertilization: Fusion of male and female gametes to form a zygote.
- Cleavage: Repeated divisions taking place in the zygote.
- Morulation: Formation of solid ball of cells.
- Blastulation: Formation of hollow ball of cells.
- Gastrulation: Movement and rearrangement of cells.
- Organogenesis: Development and differentiation of different organs from primary germ layers.
- Morphogenesis: Growth and differentiation of form and structure.
- Growth: Increase in size and weight through cell division and cell enlargement.
Sequential events in the development of frog:
1. Gametogenesis:
- Sperms and ova (eggs) are produced by respective gonads in males and females by meiosis.
- The sperms are microscopic, thread like and motile, 0.03mm in length.
- Ova are larger than sperms, non-motile and nearly spherical. Eggs are mesolecithal and telolecithal.
2. Mating:
- During breeding season (rainy season), the croaking of male attracts the female frog. Frogs copulate in water by a sexual embrace called amplexus or pseudo-copulation.
- The male comes over the back of the female and firmly clasps her body by his forelimbs.
- When the female sheds ova (several hundred) through her cloaca, the male deposits spermatic fluid over them and fertilization occurs.
3. Fertilization:
- Fertilization in frog is external, as it occurs in water.
- One sperm penetrates one ovum and as soon as the male pro-nucleus enters the ovum, a second polar body is budded off from the ovum.
- The first polar body was already present below the vitelline membrane.
- After the second maturation division giving off second polar body, the fusion of male and female nuclei takes place. This process is called fertilization.
- The cell thus formed is called zygote and the nucleus is called zygote nucleus which has diploid chromosome number, 26.
- The zygote now divides through a process called cleavage or segmentation.
4. Cleavage or segmentation:
- In frog, the division is complete but the cells formed are unequal. So, the cleavage is called holoblastic but unequal.
- The first division is vertical and divides the zygote into two blastomeres. The furrow extends from animal pole to the vegetal pole (lower end).
- Second division is also vertical but at right angle to the first resulting in 4 blastomeres.
- The third division is horizontal passing above the equator forming 8 unequal blastomeres. Out of these, 4 upper smaller ones are called micromeres and 4 lower larger ones are called megameres.
- Two more vertical divisions (fourth cleavage set) take place forming 16 cells (8 megameres and 8 micromeres).
- Two horizontal divisions following this, results in the formation of a 32-celled stage.
5. Morulation (formation of morula):
- After 32-cell stage, cleavages become less regular and difficult to follow.
- Micromeres divide more rapidly than the megameres. This is due to less quantity or lack of yolk in micromeres and megameres having more yolk present on the vegetal end.
- Due to this irregular division of cells, the dividing zygote appears like a mulberry-shaped solid ball of cells called morula.
6. Blastulation (formation of blastula):
- As the division continues, the blastomeres arrange at the periphery and a small central fluid filled cavity or space appears within an embryo, called blastocoel or segmentation cavity.
- Thus, the embryo appears as a hollow ball and is now called a blastula (coeloblastula). This process of formation of blastula is called blastulation.
- Although, blastula appears to be composed of only micro and megameres, the cells forming future parts of the body can be identified by special staining methods. The areas are:
- Animal pole of the blastula represents the presumptive ectoderm. This can further be divided into presumptive epidermis and neural plate (nervous system).
- A small area near the vegetal pole is the presumptive notochord.
- Close to notochord lies the presumptive mesoderm.
- The remainder of the vegetal pole formed by large yolk laden megameres forms the future endoderm.
7. Gastrulation (formation of gastrula):
- This is formed after blastula stage. The conversion of blastula to gastrula is called gastrulation which is completed by four processes:
a. Epiboly (overgrowth of micromeres):
- The micromeres of the animal pole divide repeatedly and spread over the lower megameres.
- Thus, the presumptive notochord, mesoderm and endoderm get enclosed leaving a small area called yolk plug.
b. Imboly:
- Behind the presumptive notochord, invagination appears which is the beginning of archenterons.
- The open end of this invagination is called blastopore. Its anterior end is the dorsal lip of the blastopore.
- As archenteron extends inwards, it becomes an extensive cavity called archenteron cavity which lies above megameres.
c. Involution or Migration of micromeres:
- Micromeres begin to migrate inwards from dorsal lip and with it the archenteron enlarges. Micromeres form a thick layer or the dorsal surface of archenteron which form the future notochord and mesoderm.
- When the development of archenteron begins, blastopore decreases in size and gradually disappears.
- The migration of micromeres also takes place on the side and on the ventral surface of dorsal lip, forming lateral lips and ventral lip respectively. These lips unite and reduce the size of blastopore. Through this blastopore come out yolk filled megameres called yolk plug.
d. Rotation of gastrula:
- Now the gastrula rotates inside the vitelline membrane. The blastopore comes near the original vegetal pole of the embryo. During this, the yolk plug moves inwards and in the ventral surface of the archenteron.
- The complete disappearance of blastocoel marks the end of gastrulation.
- After all these changes, three layers can be seen in the gastrula;
- The outer surface, which forms the ectoderm (future neural plate and epidermis )
- The cells on the roof of the archenteron called chordamesoderm.
- Floor and sides of the archenteron forming endoderm.
- Still now, chordamesoderm and endoderm are not clearly distinguishable. At the end of gastrulation, a process called neurulation takes place during which notochord is formed from chordamesoderm.
8. Organogenesis:
- Gastrulation is completed in about thirty hours after fertilization.
- The three layers of cells; ectoderm, mesoderm and endoderm formed after gastrulation are called three germinal layers (in triploblastic metazoans).
- Further development of the embryo is concerned with the formation and differentiation of various embryonic tissues from these three primary germinal layers.
- This phase of development of called organogenesis, which ultimately leads to the formation of an active free swimming larval stage called tadpole larva.
- Organogenesis involves the following various changes:
a. Neurulation (formation of neural tube):
- On the mid-dorsal region, ectoderm cells thicken to form neural plate. On either side of this are the neural folds.
- Both neural folds increase in size and fuse at the mid-dorsal region forming the neural canal or tube which opens at the anterior end by a small opening called
- Neural tube posteriorly remains connected for sometime with archenteron by neurenteric canal.
- At this stage, the embryo is called neurula.
- At the end, the neural tube is covered into a closed tubular canal, the anterior part of which is the future brain and the posterior portion forms the spinal cord.
b. Formation of notochord and mesoderm:
- At the mid-dorsal region, the chordamesoderm forms a cylindrical rod like structure which forms the notochord. The rest of chordamesoderm gives rise to mesoderm.
- On the either side of the notochord lies mesoderm which can be divided into three parts:
- Dorsally situated epimere, which further has three more parts;
- Myotomes forming body musculature
- Dermatomes forming dermis of skin
- Sclerotomes forming axial skeleton
- Middle mesoderm called mesomere or nephrotome, which gives rise to excretory and genital organs
- Ventral mesoderm called hypomere or lateral plates, which on either side divide to form layers with a narrow space in between the layers called coelom. The outer layer of mesoderm forms somatic and inner layer forms visceral layer of coelom.
- Dorsally situated epimere, which further has three more parts;
c. Formation of endoderm:
- The cells forming the floor of archenteron divide, extend dorsally and completely enclose archenteron. This layer is below the mesoderm and forms the endoderm.
- Now the embryo which has elongated to some extent can be said to have three primary germinal layers namely ectoderm on the outer side, endoderm on the inner side and mesoderm in between them.
Fate of three germinal layers:
- Ectoderm gives rise to:
- Epidermis and cutaneous glands
- Lining of the cloaca and mouth cavity
- Central nervous system, brain and spinal cord
- Lens, cornea and retina of the eye
- Olfactory and auditory organs
- Endoderm gives rise to:
- The epithelium lining the digestive canal except mouth and cloaca, and digestive glands like liver and pancreas
- Larynx, trachea and lungs of the respiratory system
- Lining of the urinary bladder
- Thymus and thyroid glands
- Mesoderm gives rise to:
- Dermis of the skin
- Cartilage and bones of the skeletal system
- Blood vascular system including blood and blood vessels
- Excretory and genital organs
- Spleen, sclerotic and choroid of the eye
9. Morphogenesis:
- Further development of elongated embryo occurs leading to the pre-tadpole stage followed by the larval stage.
a. Pre-tadpole stage:
- After about 4 days of fertilization, the embryo becomes about 4mm long and lies within the egg membrane.
- The body can be divided into head, trunk and tail.
- On the head, on either side lie the round elevations showing the position of future tympanum.
- On the ventral side of the anterior end is a U-shaped sucker (cement gland) formed by mucus gland cells.
- Between sucker and nasal pit is a small depression forming At the posterior end is another depression called proctodaeum.
- Posterior to proctodaeum, the body elongates to form the tail. Internally, the embryo contains parts of central nervous system, notochord, closed alimentary canal, liver, heart and rudiment of urinary bladder.
- After development of such organs, it is time for embryo to hatch out.
b. Hatching:
- About 2 weeks after fertilization, the embryo becomes about 6mm in length.
- It breaks out the egg membrane and comes out.
- This is called hatching and the free larval stages of the frog after hatching are called tadpoles.
c. Tadpole larva:
- The newly hatched tadpole is a small, blackish, fish-like creature.
- In the beginning, the tadpole gets attached to some aquatic plant by means of its ventral sucker or cement gland.
- First two pairs of external gills develop on either side of the head. Soon they develop into three pairs of branched gills, and help in respiration along with skin.
- The tadpole in its first week of hatching gets nourishment from the yolk still present in the cells of archenteron.
- The mouth develops 7 days after hatching, which is bound by two horny jaws after which it feeds on aquatic plants.
- Stomodaeum and proctodaeum get connected with gut forming a complete alimentary canal. It is at first small and broad but later becomes long and coiled like a spring.
- Slowly three pairs of external gills are replaced by four pairs of internal gills. Internal gills are covered with an operculum.
- Mesonephric kidney of adult begins to appear.
- Lateral line system is well-developed.
- Fore limbs and hind limbs start forming. Fore limbs develop slowly than hind limbs as they remain excluded by operculum.
- Lungs begin to appear and when fully formed, a tadpole respires through both lungs and gills.
10. Metamorphosis and growth:
- Tadpole larva completely differs from the adult frog in form and nature.
- Two or three weeks after breathing through lungs, the tadpole undergoes drastic changes to transform into a small young frog.
- The changes that transform a tadpole into an adult frog are called metamorphic changes and the process is called metamorphosis.
- It includes morphological, physiological and behavioral changes.
- Before metamorphosis, the tadpole develops a thyroid gland which secretes the hormone, thyroxine essential for metamorphosis.
- Some of the important changes are as follows:
- The tadpole stops feeding.
- The tail tissue forms a nutritive substance which through blood provides nutrition to the larva.
- The tail begins to disappear slowly.
- The sectorial mouth becomes wider and a large sticky tongue is developed.
- The eyes increase in size.
- The forelimbs come out of the operculum and the hind limbs become longer.
- Skin becomes vascular, glandular, respiratory and pigmented.
- The lateral line system disappears.
- In addition to the above external changes, the following internal changes also take place.
- Gills and operculum completely disappear.
- Lungs become more functional as respiratory organs.
- Cartilaginous skeleton is replaced by bony endoskeleton.
- Middle ear and tympanum develop.
- Stomach and liver enlarge.
- The feeding habit changes from herbivorous to carnivorous and the long coiled intestine shortens in length.
- The young frog has a stumpy tail.
- It leaves water to live in damp places on land. It feeds on insects and grows into adult leading an amphibious life.