In zoology, an egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing embryo can survive on its own.
The term “egg” is used differently outside the animal kingdom, for an egg cell. Reproductive structures similar to the egg in other kingdoms are termed spores, or seeds. Animal eggs are as different and varied in their structure as the animals who made them.
Today in this I shared with you excellent list of various egg in brief. Take a look….
Amphibians are members of the class Amphibian, a group of vertebrates whose living forms include frogs, toads, salamanders, newts and caecilians. They are characterized as non-amniotic, ectothermic tetrapods, meaning their eggs are not surrounded by membranes, they are cold-blooded, and they have four limbs.
Most amphibians lay their eggs in water and the larvae undergo metamorphosis from a juvenile form with gills to an adult air-breathing form with lungs. Some, however, are paedomorphs that retain the juvenile water-breathing form throughout life. Mudpuppies and olms are examples of this, retaining juvenile gills into adulthood. Adult amphibians also use their skin for respiration.
The three modern orders of amphibians are placed in the subclass Lissamphibia and are the Anura (frogs and toads), Caudata (salamanders and newts) and Gymnophiona (caecilians, limbless amphibians that resemble snakes). The total number of known amphibian species is approximately 6,500.
They are superficially similar to reptiles, but reptiles, along with mammals and birds, are amniotes, having impervious membranes that surround the egg. Amphibians are ecological indicators,
and in recent decades there has been a dramatic decline in amphibian populations around the globe. Many species are now threatened or extinct. The study of amphibians is called batrachology.
Most amphibians lay eggs that have a gelatinous coating which swells when it comes in contact with water. The larvae that hatch from the eggs are mostly quite dissimilar to the adult form. In the case of frogs and toads they have a large head and a dorsally flattened tail and are known as tadpoles. They are vegetarians and breathe with gills.
They have no limbs at first, the back limbs thrusting through then skin at a later stage, followed by the fore limbs after which the tail is reabsorbed. After this metamorphosis the juveniles look like miniature versions of the adult.
Newt and salamander larvae have long bodies and feathery gills. They are carnivorous and the front legs develop before the back ones. They do not undergo metamorphosis in the same way that frogs and toads do.The caecilians either produce live young or lay eggs in damp positions in their burrows.
The earliest amphibians evolved in the Devonian Period from Sarcopterygians, fish that had lungs and bony fins, features that were helpful in adapting to dry land. They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrunk and decreased in diversity, leaving the modern orders of Lissamphibia.
Sponge and Jelly Eggs
Sponges, jellies, and corals produce eggs in a similar method to most fish. They do not have males and females. As an alternative, simple male and female organs both arise on a single creature, which discharge eggs and sperm into the water. Several reproduce asexually, not including even the male and female organs, by simply discharge some of their cells to grow directly into new individuals without needing to be fertilized.
Some sponges and jellies can have children if they are broken up into pieces, where each piece broken off them grows into a new individual. In some species of sponges, if you were to slice one up finely and spread out the fragments, they would merge back together and reform. If you spread them out far enough, each fragment would simply grow into a new sponge.
Dinosaur eggs are represented today as fossils. They represent the product of egg laying activity and can offer clues as to how dinosaurs behaved.In some cases the embryonic dinosaur is preserved within the eggshell, and can be studied.
Dinosaur eggs are known from about 200 sites around the world, the majority in Asia and mostly in terrestrial (nonmarine) rocks of Cretaceous Age. It may be that thick calcite eggshells evolved during the Cretaceous (145 to 66 million years ago). Most dinosaur eggs have one of two forms of eggshell that are distinct from the shells of related modern animal groups, such as turtles or birds. However, some dinosaur eggs closely resemble bird eggs, particularly the type of eggshells in ostrich eggs.
The egg structure consists of a series of basic vertical units that grow from particular sites on the surface of the shell. The organisation of these units determines the classification scheme, being either spherulitic or prismatic:
- Spherulitic egg shells show spherical patterns in the crystalline structure, and they are seen in sauropods and hadrosaurs.
- Prismatic egg shells grow into spherical crystals only in the lower portion of the shell, while crystals in the upper portion are prisms.
- Ornithoid eggs (also seen in birds) are generally laid by theropods. In this type only the very bottom part of the shell exists as separate or discrete units (mammilae). The upper and mid-portions of the shell consist of a mass of biocrystalline material with a spongy (squamatic) ultrastructure, that comprises a homogeneous layer.
The shape of dinosaur eggs may reflect some aspects of their biology. In general, they are more symmetrical than bird eggs, implying that the head was not given more priority than other structures during development. Like birds, the shell was likely an important source of calcium, especially later in development. In modern reptiles, calcium from the shell is mobilized and stored in the yolk sac immediately prior to hatching.
A spherical egg would have provided the smallest shell surface compared to the volume of the egg but many dinosaur eggs were elongated. This shape would have increased the total volume of an egg given the diameter of the passage between the pubic bones.
It would also have greatly increased the area of calcium-rich shell available to the embryo and may have been important in animals producing precocious young with large well-developed legs and a long bony tail.
The oldest known dinosaur eggs are from Massospondylus, which lived during the Early Jurassic, about 190 million years ago.
Octopuses thread their soft, translucent eggs up on overhangs of rock or coral. The females lay hundreds of thousands of eggs at a time and will stay to protector them alongside hungry predators awaiting they hatch. This often takes so long that she begins to starve and some octopuses will eat their own arms to stay alive. Once her thousands of tiny offspring are hatched, they feed on microscopic organisms like plankton until they grow large enough to live on the sea floor as adults. The mother, often disabled, will generally be eaten by a predator once she leaves her lair because she has grown too weak to defend herself.
Feminine insects often accumulate sperm from a particular mating to use for every consequent fertilization, so many males die after their only companion. Insects will lay many eggs at once, and sometimes assemble overgenerous nests or nurseries for them. The eggs themselves can be attractively shaped or masked. Some eggs are laid in water and the newborn insects are adapted to spending the first portion of their life aquatically before emerging into the air. Many insects will care for their eggs after they are laid, with some ants and termites even controlling the humidity and pH for them.