Develpoment of frog

Development of frog

Embryology is the study of development of animals. It deals with the changes in fertilized egg to become an adult. The stages of embryonic development differ in different chordates but help early stages and basic steps of development and similar in all.

Fertilization and development of egg of frog

Sequence of events before and during the embryonic development of frog includes

i) Gamatogenesis: formation and differentiation of sex cells or gametes.

ii) Fertilization: fusion of male and female gametes to form zygote.

iii) Cleavage: division of zygote

iv) Blastulation: formation of hollow ball of cells.

v) Gastrulating: Movement arrangement of embryonic cells to form 3 primary germ layers.

vi) Organogenesis or organ formation: Differentiation and development of organ system from primary germinal layers.

vii) Differentiation and morphogenesis: Acquisition (thing acquired) of morphological feature of the organism.

Vii) Growth: increase in size through the development.

Mating behavior (June to September)

·         Vocal sac and pads involve

·         Ovipositon by female

·         Insemination by male 

Eggs of Rana tigrina

  Male produces a loud crocaking sound at night, so many times to attract the female. Copulation occurs in water. Male mounts on backs of the female and holds it firmly with its coupulatory pad developed on the index finger of forelimbs. During copulation the male remains inactive and female swims. The sperms dissolve the vitelline and plasma membrane of the ovum by proteolytic enzymes called sperm lysine secreted by its acrosome. Small cone of reception is formed at place from where the sperm enters on the surface of ovum.

  As the sperm enters following changes occur in ovum. (Fertilization)

1.        Cone of reception is with drawn.

2.        Some water oozes out from the cytoplasm and collects between its vitelline and plasma membrane. The vitelline membrane is separated from the main cell by the thin fluid.

3.        So that ovum can rotate inside. Freely and animal hemisphere comes to lie on the upper side in water and vegetal hemisphere lies on the lower side.

4.        Mitosis in ovum is completed a second polar body is given off close to the first polar body.

5.        Pronucleus of sperm reaches the pronucleus of egg and fused which results the formation of the zygote nucleus. Two centrioels of the sperm more further apart by this time and from a division spindle for first cleavage.

6.        During entry of sperm some pigments and water are carried into the zygote from the point of penetration. Which forms a grey colour called grey crescent appears on the surface of zygote.

Early embryonic development or development of eggs

The fertilized egg undergoes series of mitotic division the development of egg up to the formation of three germinal layers includes the following stages.

I)                    Cleavage

II)                  Morulation

III)                Blastulation

IV)                Gastrulation

I) Cleavage or segmentation.

1)       Size of egg becomes 1.6mm after 2.3hrs of fertilization and cleavage starts all cleavage are mitotic and holoblastic.

2)       First division is vertical which begins as small furrow at the animal pole and finally surrounds the eggs. It makes two equal blastomeres.

3)       Second division is also vertical but night angles to the first one from four blastomeres.

4)       Third division is horizontal passing above the equatorial plane forming 8 unequal blastomeres. Upper four are smaller, known as micromeres and lower four cells are larger called megameres. (4 to 5 hrs)

5)       Fourth and fifth (after 20 minutes of 3^rd cleavage) are again vertical from 16 cells. 8 are micromeres and 8 are megameres.

6)       Two horizontal  6^th and 7^th division produce 32 cells. After 32 cells stage the division becomes less regular.

II)                  Morulation: it is a mulberry shaped ball of cells due to rapid division of micromeres than megameres.

III)                Blastulation: Due to rapid and non synchronizing cleavage of micromeres and megameres a cavity appears in the center known as blastocoel and stage called blastula. The division of blastomeres and gets filled with a fluid.

Primary germ layers viz. ectoderm, mesoderm and endoderm are formed. Various organs are formed from there layers. Hence, there parts are called presumptive or prospective areas (fate map)

These areas changes into

a.        Ectoderm changes into epidermis and neural tube.

b.       A small area near vegetal pole is presumptive notochord.

c.        Mesoderm lies side of notochord.

d.       The remainder of the vegetal half forms the future endoderm.

IV)                Gastrulation:  in this stage following process takes place.

                                 

 a.   Epibody: Micromere cells of animal’s pole divide and spreading over the lower megamere cells. Small area uncovered by micromeres on vegetal pole called yolk plug.

b.        Imboly or invagination

·         Invagination (start of archenteron) is formed due to migration of micromeres below the grey crescent.

·         Archenteron becomes larger and larger and reduces the blastocoel and disappeared.

·         Dorsal lip of blastopore is formed from where the involution (turning or rolling inward of cells) starts.

·         Wall gets thinner at grey crescent area due to migration of cells.

·         Micromeres grow and yolk cells from the boundary of archenteron.

·         The boundary lie below the ectoderm called meso-endoderm.

c.        Contraction of lips of blastopore:

·         Dorsal, ventral and lateral lips of blastospore are formed.

·         Dorsal and ventral lips contract and decreses the size of blastospore.

·         Due to the contraction of  some macrome cells bluge out from blastopore from yolk plug.

·         This stage of embryo is called yolk plug stage.

·         Ectoderm ( future neural plate and epidermis)

                                                                                                                                                                     Chordamesoderm and endoderm are formed.

                        Significance of gastrulation

i.                     Cells from different layer to form organs.

ii.                    The layers changes to from different structure (morphogenesis )

iii.                  Increased metabolic activities.

V)                  Formation of three germ layer

·             Outer of layer the embryo formed by micromeres called ectoderm.

·             Inner layer of the embryo formed by micro and megameres from roof of archenteron called meso-endoderm.

·             Mesoderm is formed by separating by splitting of cells from endo mesoderm. Splitting starts on dorsal surface.

·             Simultaneous endoderm is formed.

Following organs are derived from the layers.

Ectoderm	Mesoderm	Endoderm
1)       Epidermis and epidermal gland. 2)       CNS 3)       Pituitary gland, pineal gland adrenal gland etc.	1)       All muscular tissue. 2)       Coelom 3)       Pericardium notochord, most skeleton etc.	1)       Mucous membrane of gut. 2)       Middle ear 3)       Trachea bronchi and lungs etc

  VI)            Postgastular stage and organogeny

                Gatrulation is completed in about 30 hours after oviposition. Different tissues and embryonic organs are formed from the three primary germ layers.

a.        formation of notochord (notogenesis):

·         notochord develop from the mesodermal layer of archenteron.

·         Mesodermal cells from notochord, mesoderm and endoderm.

·         Cells of roof arechenteron become vaculated and a sheath is formed around vaculated cells.

·         Later the notochord is replaced by vertebral column.

b.        Formation of nuron tube ( neurulation):

·         By the end of gatrulation, the neural palte appers plate appers by thickening of ectoderm cells.

·         Lateral plate appears by thickening of ectoderm cells.

·         Neural plate sink downward to from a neural groove. The folds joim and neural tube is formed.

·         Cavity of neural tube (neuro coel) called central cannal of brain and spinal cord

·         Fornt end of neural fold form a neuropore.

·         Neural folds merge posteriorly and enclose the blastopore.

·         From neuropore to the previous but now closed blastospore from a cannal which is called neuroentric cannal.

·         Anterior borad end of neural tube modifies into brain and the neural tube from medulla.

e.        Formation of coelom :

·         Coelom is formed by mesoderm.

·         A split occurs in the mesoderm and outer in the mesoderm and outer paritel or somatic mesoderm ( below the skin ) and an inner visceral or splanchnic endoderm is formed around the wall of archenteron.

·         The cavity thus formed between the layer is called splanchno coel (future coelom)

·         Splanchnocoel extended downward below the gust to apper v-shaped in section.

·         Visceral layer (which cover viscera or different organs) united with endoderm to form the get wall or splanchnopleure. C. wall of oesophagus intestine rectum etc.