The development comprises four main processes:
- Increase in the number of cells by mitosis (cleavage)
- Arrangement of the cells into embryonic layers (gastrulation)
- The formation of different organs and systems from the embryonic layers (organogenesis)
- The increase in size of the body during development (growth)
Cleavage
After a period of 2-3 hours following fertilization, the fertilized egg starts dividing into two cells by an axis passing from animal pole to the vegetal pole. Each cell is called blastomere. The second division or cleavage appears about one hour after the first cleavage, at right angles to the first on thus giving rise to four blastomeres. The 3rd cleavage starts after about another half an hour and is horizontal but slightly above the equator. The embryo now has eight blastomeres. The fourth cleavage appears after about still another 20 minutes and divides the cells vertically. After this the cleavage becomes rapid and a large number of blastomeres of unequal sizes are formed, the upper cells dividing more rapidly than the lower ones. This cleavage or segmentation results in the formation of a solid ball of cells, which is called mourla.
Blastulation
Soon after the formation of morula, a series of changes starts that leads to blastulation. A cavity appears due to compact rearrangement of cells within the embryo in the upper half, called blastocoels. The embryo at this stage is called blastula and is a hollow ball of cells. Now the cells move to occupy their new position according to the function they have to perform.
Gastruation
The formation of blastula leads to the movement and rearrangement of cells to occupy their definitive position. The smaller pigmented cells spread over the large yolk cells, except a small circular area called yolk plug. This is the area where a circular depression called blastospore starts appearing. The cells along the margin of the blastopore move inward (in-fold) to form the mesoderm and endoderm. The mesoderm formed behind the dorsal lip gives rise to the notochord. The ectoderm gives rise to the outer layer of skin and nervous system. The endoderm gives rise to digestive and some part of respiratory systems. All the other systems are formed from mesoderm. In the meantime, a cavity called archenteron appears, enlarges and becomes lined with the endoderm. At this stage the embryo is called gastrula and all the changes which result in the conversion of blastula into gastrula are collectively known as gastrulation.
Neurula
The dorsal ectoderm differentiates into the neural tube to form the nervous system. This process is called neurulation. At this stage the embryo becomes elongated, develops cilia, swims freely, and is called neurula.
Tadpole
After neurlation rapid changes occur in the different cellular layers to form different organs and systems of the body that convert the embryo into the larva called tadpole. The tadpole is apparently fish-like in shape having external gills, internal gills and a long tail. It swims by undulation of its body.
When tadpole larva is formed, the stored food in the form of yolk has been consumed and the larva starts feeding on vegetation.
Metamorphosis
After an active free swimming phase, the tadpole passes through a series of rapid changes that transforms it into the tailless adult frog. The process involving changes that transform larva into adult is known as metamorphosis. The average duration of development of frog is 70-90 days.
- Egg + Sperm (fertilization) Zygote
- Zygote (rapid cell division or cleavage) Morula
- Morula (formation of blastocoels) Blastula
- Blastula (re-arranging of cells and formation of 3 germinal layers) gastrula gastrulation
- Gastrula (elongation and formation of neural tube) Neurula
- Neurula (formation of tail and gills) Tadpole
- Tadpole (metamorphosis) Frog
A summary of development of frog
Fig 16.18 Life cycle of frog showing different developmental stages