the disappearance of animal
the disappearance of animal
DEVELOPMENT OF FROG
BREEDING IN FROG
Occurs during monsoon period from july to
Occurs in shallow water.
Osmoreceptors, in mouth make their way to
Croacking by males attract females.
Male recognizes mature female by swollen
Male clasps female, ovulation occurs and
simultaneously sperms are shed.
Characteristics of amphibians in which pairing of sexes
occur and both the gametes are discharged into water.
Laying eggs by female frog stimulated by amplexus
Spawn:- eggs laid in irregular mass in water.
Sperm:- microscopic about
0.03mm, consist of head,
middle piece and tail.
from golgi apparatus) and
and mitochondria in
Tail:- contains axial
filament and cytoplasm.
DETAILED VIEW OF SPERM
Telolecithal and mesolecithal.
Released from ovary in
primary oocyte state.
Enclosed by vitelline
Coloured half called animal
White half called vegetal
hemisphere contains york.
60% protein and 40% fat.
Occurs in water and is external.
Sperm enters by animal pole dissolving plasma and
At point of entry cone of reception is formed.
Penetration path is formed.
Copulation path by sperm head to nucleus is formed.
After sperm entry second polar body is released and egg
changes into ovum.
Pronuclei of both gametes approach and have haploid
number of chromosomes.
Chromosome of both nuclei come together and arrange on
Ovum with diploid no of chromosome is called zygote.
ENTRY OF SPERM INTO
FUSION OF SPERM AND EGG
Mitotic divisions of zygote upto many celled embryo after
two hours of fertilization is HOLOBLASTIC (cleavage
furrow divides zygote and its descendants completely)IN
FIRST CLEAVAGE:-It is meridional from animal to vegetal
pole and divides zygote into 2 blastomeres.
SECOND CLEAVAGE:-It is also vertical but at right angle
to first and 4 blastomeres are formed.
THIRD CLEAVAGE:-Is horizontal and 8 celled stage is
formed having two tiers 4 lower macromeres and 4 upper
micromeres i.e. DIFFERENTIAL CLEAVAGE.
A solid ball called morula is
At 8-16 celled stage blastocoel
or segmentation cavity
Blastocoel increases by
secretion into it an albuminous
material from surrounding
cells and absorption of water
Formation of hollow, spherical
embryo, blastula, wall known
Blastocoel eccentric in
presumptive areas of
blastula give rise to
specific parts of future
Egg is indeterminate.
Plasticity is lost sooner.
Cells of dorsal lip of
blastopore are irreversibly
It acts as an organizer.
OVERVIEW OF FATE MAP
Migration of the various presumptive areas of
single layered blastula to their proper positions in
Mass migration of cells is morphogenetic
Cell division does not occur.
Rearrangement of cells to specific positions
occur in gastrulation.
MECHANISM OF GASTRULATION
Epiboly – anterior cells of animal half start
spreading backwards over large yolky cells of
Involution and invagination – a small groove
appears in middorsal region at the posterior
Micromeres roll over anterior edge and move
towards beneath outer layer.
Formation of archenteron – crescentic groove
formed by invagination of macromeres marks the
beginning of cavity archenteron or gastrocoel.
INITIATION OF GASTRULATION
Opening of archenteron is known as blastopore.
Completion of blastopore - anterior margin of
blastopore is dorsal lip.
1. The ends of blastopore grow backwards and form
2. The lateral lips meet on ventral side forming
3. Heavy yolk laden megameres form the yolk plug.
Elongation – embryo slowly elongates, mesoderm
splits as a sheet from endoderm.
Delamination starts dorsally and proceeds
GASTRULATION IN FROG EMBRYO
Rotation – movement of heavy megameres from
posterior to ventral position, disappearance of
blastocoel and formation of dorsal archenteron,
cause a slow shift in center of gravity.
The embryo is now called gastrula.
About 2.5mm long
Three primary germ layers are formed viz.,
Ectoderm, mesoderm and endoderm.
POST GASTRULAR DEVELOPMENT
ORGANOGENY – Differentiation of germ layers into
Neural plate gives rise to neural folds and form
The neural folds meet in middle line and form neural
tube with a cavity neurocoel.
At anterior end neural tube opens to exterior by
Communicates with archenteron by neurentric canal.
Narrow strips, the neural crests on the sides give rise
to dorsal root ganglia of cranial and spinal nerves,
sympathetic ganglia and suprarenal glands.
Presumptive notochord lies as a narrow band in
roof of archenteron.
It gets separated from mesoderm by narrow
clefts by delamination.
The notochordal cells arrange themselves in the
form of rod and become vacuolated.
Sheaths develop around the rod to complete
Mesoderm lying on sides of notochord thicken and
become segmented, forming blocks called epimeres
or mesodermal somites or vertebral plates.
The rest of mesoderm remains unsegmented and
forms hypomere or lateral plate mesoderm.
Between epimere and hypomere, there is a
thickened segmented ridge of mesoderm known as
mesomere or nephrotome.
Gut develops from the archenteron.
The yolk plug is withdrawn into floor of archenteron
and blastopore is closed by ectoderm.
Foregut – formed in front of yolk mass –
differentiates into pharynx, oesophagus, stomach.
Midgut – formed of yolk mass – differentiates into
Hindgut – formed behind yolk mass – differentiates
into rectum and cloaca.
Ectodermal invagination below position of
blastopore forms the proctodeal rudiment.
Gives rise to cloacal aperture
Ectodermal invagination on lower side of anterior
end forms the stomodaeal rudiment.
Gives rise to mouth and buccal cavity.
Outgrowths from gut gives rise to many organs –
thymus, liver, pancreas, thyroid, urinary bladder,
The region behind the proctodaeum grows into a
A horseshoe-shaped glandular pit, called sucker
rudiment grows behind stomodaeal rudiment.
Rudiments of sense organs also appear.
Heart is formed from splanchnic mesoderm below
Two pairs of external gills arise on side of anterior
Growth in length, changes in shape and
organogenesis finally change the embryo into a
larva ready for hatching.
Embryo softens and dissolves the jelly coat
and vitelline membrane by certain enzymes
It wriggles out and known as tadpole larva.
Hatching occurs after 24 hours after
Fertilization in Rana tigrina.
NEWLY HATCHED TADPOLE
EXTERNAL CARACTERS –
1. About 6mm, blackish fish like creature.
2. Body divided into three regions- head, trunk
3. Head bears nasal pit, ear rudiment, eye
rudiment and pair of external gills.
HABITS – The newly hatches larva swims for
sometime and then fixes itself o some objects.
It does not feeds and subsists on yolk cells
heaped in its gut.
GROWTH OF TADPOLE
After 2 days, proctodaeum opens into gut, forming cloacal
After 3 days stomodaeum develops connection with gut,
Mouth is small, acquires horny jaws, which bears horny
papillae or teeth.
Now tadpole starts feeding.
The external gills grow in size and help in gaseous
The visceral pouches develop internal gills.
The suckers disappear.
External gills shrivel and fall off.
Changes occur in vascular system for breathing
with internal gills.
Tail elongates, acquires tail fin.
Lens and cornea formed, eyes become functional.
Lungs arise as outgrowths behind pharynx.
Lateral line organs are formed.
A fold of skin covers external gills, forming gill
cover or operculum.
The notochord is the only supporting structure and
the cartilaginous skeleton is laid down gradually
with appearance of limb buds in the life of tadpole.