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DEVELOPMENT OF FROG BREEDING IN FROG Occurs during monsoon period from july to september. Occurs in shallow water. Osmoreceptors, in mouth make their way to water. Croacking by males attract females. Male recognizes mature female by swollen abdomen. Male clasps female, ovulation occurs and simultaneously sperms are shed. AMPLEXUS Characteristics of amphibians in which pairing of sexes occur and both the gametes are discharged into water. SPAWNING Laying eggs by female frog stimulated by amplexus with male. Spawn:- eggs laid in irregular mass in water. GAMETES Sperm:- microscopic about 0.03mm, consist of head, middle piece and tail. Head:- acrosome(derived from golgi apparatus) and nucleus(condensed D.N.A.) Middle piece:-centrioles and mitochondria in cytoplasmic sheath. Tail:- contains axial filament and cytoplasm. DETAILED VIEW OF SPERM EGG Telolecithal and mesolecithal. Released from ovary in primary oocyte state. Enclosed by vitelline membrane. Coloured half called animal hemisphere contains nucleus. White half called vegetal hemisphere contains york. 60% protein and 40% fat. FERTILIZATION Occurs in water and is external. Sperm enters by animal pole dissolving plasma and vitelline membrane. 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 spindle. Ovum with diploid no of chromosome is called zygote. FERTILIZATION PROCESS ZY ENTRY OF SPERM INTO EGG FUSION OF SPERM AND EGG CLEAVAGE Mitotic divisions of zygote upto many celled embryo after two hours of fertilization is HOLOBLASTIC (cleavage furrow divides zygote and its descendants completely)IN FROG. 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. BLASTULATION A solid ball called morula is formed. At 8-16 celled stage blastocoel or segmentation cavity appears. Blastocoel increases by secretion into it an albuminous material from surrounding cells and absorption of water from outside. Formation of hollow, spherical embryo, blastula, wall known as blastoderm. Blastocoel eccentric in position. FATE MAP Particular areas, presumptive areas of blastula give rise to specific parts of future embryo. Egg is indeterminate. Plasticity is lost sooner. Cells of dorsal lip of blastopore are irreversibly determined. It acts as an organizer. OVERVIEW OF FATE MAP PRESUMPTIV E AREAS PRESUMPTIV E ECTODERM ANTERIOR PRESUMPTIVE EPIDERMIS PRESUMPTIVE NOTOCHORD PRESUMPTIVE NEURAL PLATE PRESUMPTIVE MESODERM SOMITIC MESODERM LATERAL PLATE MESODERM PRESUMPTIVE ENDODERM POSTERIOR PRECHORDAL ENDODERM GASTRULATION Migration of the various presumptive areas of single layered blastula to their proper positions in gastrula. Mass migration of cells is morphogenetic movement. 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 vegetal half. Involution and invagination – a small groove appears in middorsal region at the posterior margin. 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 lateral lips. 2. The lateral lips meet on ventral side forming ventral lips. 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 ventrally 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 ORGANOGENY organs. NEUROGENESIS FORMATION OF NEURAL TUBE NOTOGENESIS FORMATION OF NOTOCHORD MESOGENESIS DIFFERENTIATION OF MESODERM ENTEROGENESIS DIFFERENTIATION OF GUT NEUROGENESIS Neural plate gives rise to neural folds and form neural groove. The neural folds meet in middle line and form neural tube with a cavity neurocoel. At anterior end neural tube opens to exterior by neuropore. 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. NEURALATION NOTOGENESIS 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 notochord. MESOGENESIS 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. MESODERM LATERAL PLATE MESODERM FORMATION OF TRUE COELOM, SOMATOPLEUR AND SPLANCHNOPLEUR SOMITIC MESODERM NEPHROTOME DERMATOMES SEGMENTAL EXCRETORY TUBULES SCLEROTOMES NEPHROCOEL MYOCOEL ENTEROGENESIS 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 intestine. Hindgut – formed behind yolk mass – differentiates into rectum and cloaca. ENTEROGENESIS 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, lungs, larynx. The region behind the proctodaeum grows into a tail bud. 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 pharynx. Two pairs of external gills arise on side of anterior region. Growth in length, changes in shape and organogenesis finally change the embryo into a larva ready for hatching. HATCHING Embryo softens and dissolves the jelly coat and vitelline membrane by certain enzymes from skin. 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 and tail. 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 aperture. After 3 days stomodaeum develops connection with gut, forms mouth. 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 exchange. 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. THANKYOU…