Introduction: Definitions:

During embryoic development, cells specialize in carrying out particular functios. In all animals except sponges, the process is irreversible: once a cell deifferentiates to serve a function, it and its descendants can never serve any other. A sponge cell that had specialized to serve one function (such as lining the cavity where feeding occurs) can lose the special attributes that serve that funciton and change to serve another function (such as being a gamete). Thus a sponge cell can dedifferentiate and redifferentiate. Cells of all other animals are organized into tissues, each of which is characterized by cells of particular morphology and capability. The fact that all other animals differnetiate irreversibly suggests that organisms with bodies contining cells specialized to serve particular functions may have an advantage over those with cells that potentially have multiple functions.

In the process of embryonic development, the cells of mot types of animals organize into three layers (called germ layers): an otuer extoderm, an inner endoderm and an intermediate mesoderm.

Blastula: Shortly after fertilization, an animal zygote first undergoes a series of mitotic divisions, called cleavage, which produces a ball of cells, the blastula.

Blastopore: In most naimals, the blastula fold inward at one point to form a hollow sac with an opening at one end called the blastopore.

Endothermy: Mammals are endothermic, an adaptation that has allowed them to be active at any time of the day or night. Also, more efficient blood circulation provided by the four chambered heard and more efficient respiration provided by the diaphragm make possible the higher metaboix rrate on which endothermy depedns.

Gastrula: An embryo at this stage is called a gastrula. The subsequent growth adn movement of the cells of the gastrula differ form one group of animals to another, relfecting the evolutionary history of the group.

Larva: Embryos of most kinds of animals develop into a larva, which looks unlike the adult of the species, lives in a different habitate, and eats different sorts of food. In most groups it is very small. A larva undergoes metamorphosis, a reorganization, to transform into the adult body form.

Mammals: Most mammals are herbivores,e ating mostly or only plants. Cellulose forms the builk of a plant’s body and is a major source of food for mammalian herbivores. Mammals do not have the necessary enzymes, however, for breaking the links between glucose molecuels in cellulose. Herbivorous mammals rely on a mutualistic partinership with bacteria in their digestive tracts that have the necessary cellulose splitting enzyems. Mammls such as cows have huge, four chambered fermentation vats derived form esophagus and stomach. The first chamber is the largest and holds a dense population of cellulose digesting bacteria. Rodents, horses, rabbits and elephants, by contrast, have relatively small sotmachs and instead difgest plant material in their alrge intestine, like a temrite. Even with these complex adaptations for digesting cellulose, a mouthful of plant is less nutritious than a mouthful of meat. Herbivores must consume alrge amounts of plant material to gain sufficient nutrition.

Placenta: In most mammla species, females carry their developing young internally in a uterus, nourishing them through the placenta. The placenta is a specialized organ that brings the bloodstream of the fetus into close contact with the bloodstream of the mother. Food, water and oxygen can pass across and wastes can pass over to the mother’s blood and be carried away.

Primates: are the mammalian group that gave rise to human species. In general, humans and chimpanzees exhibit a level of genetic similartiy normally found between closely related species of the same genus. In fact, there is a 99% sequence similarity between chimpanzees and humans.

Tissues: The cells of all animals except sponges are organized into structural and funcitonal units called tissues –collections of cells that are specialized to perform specific tasks.

Development of Particular Types of Animals:

Sponges:

As is true of many marine invertebrate animals, larval sponges are free swimming. After a sponge larva attaches to an appropriate surface, it metamorphoses into an adult and remains attached to that surface for teh rest of its life.

A unique feature of sponge cells is theri ability to differentiate form one type to anotehr, and to dedifferentiate from a specialized state to an unspecialized one. If a sponge is put thorugh a fine sieve or coarse cloth so that the cells are separated, they will seek each other out and reasemble the entire sponge -a phenomenon that does not occur in any other animal.

Flatworms:

Like sponges, a flatworm lacks a circulatory system for teh transport of oxygen and food molecules. The thin boyd of a flatworm allows gas to diffuse between its cells and the surrounding enviornment (oxygen diffuses in and carbon dioxide diffuses out).

The reproductive systems of flatworms are coplex. Most are hermaphroditic, each individual containing both male and female sexual strcuture. In most freshwater faltworms, fertilized eggs are laid in cocoons strung in ribbons and hatch into miniature adults. In constast, some marine species pass thorugh a larval stage, the fertized egg undergoing spiral cleavage, and the embryo giving rise to a larva that swims or drifts until metamorphosing into an adult, at which point it settles in an appropriate habitate.

Flatworms are known for their regenreative capacity: when a single individual of some species is divided into two or more parts, an entirely new flatworm can regrow what is missing from each bit.

Reptiles: Many lizards, including anoles, skinks and geckos, ahve the ability to lose their tails and then regenerate a new one. This ability allows these lizards to exape from predators.

Birds: Modern birds lack teeth and have only vestigial tials, but they still retain many reptilian characteristics. For instance, birds lay aminiotic eggs. Also, reptilian scales are present on the feet and lower legs of birds. Feathers are modified reptilian scales made of keratin, jsut like hair and scales. Feathers provide lift for flight and conserve heat. Feathers develop from tiny pits in the skin called follicals.

The revved-up metabolism need to power flight requires efficient blood circulation, so taht the oxygen captured by the lungs can be delivered to the flight muslces quickly. In the heart of most living reptiles, oxygen rich blood coming from the lungs mixes with oxygen poor blood returning from the body becasue the wall dividing the ventricle into tow chambers is not compelte. In birds, the wall dividing the ventricle is complete, and the two blood cirulations do not mixe. So flight muscles receive fully oxygenetaed flood.

Mammals: There are about 4000 living species of mammals which is less than the number of fishes, amphibians, reptiles, or birds. Most large, land swelling veterbrates are mammals. Mammals are distinguished form other classes of verterates by hair and mammary glands.

–Bats: are the only mammals capable of powered flight. Like the wings fo birds, bat wings are modified forelimbs, but bats have developed wings by modifying theirforearms in different ways.

Circulatory System:

Open circulatory system:

In an open circulatory system, the blood passes from vessels into sinuses (open areas within the body), mixes with body fluid that bathes the cells of tissues, then reenters vessels in another location.

Closed circulatory system:

In a closed circulatory system, the blood flows entirely within blood vessels, so it is physically separated for other body fluids. Blood moves thorugh a closed circulatory system faster and more efficiently than it does thorugh an open system.

Development of Plants:

Meristems: Meristems are clusters of small cells with dense cytoplams and proportinately large nuelei taht act as stem cells. One cell divides to give rise to two cells, of which one reamins meristematic, while the other udnergoes differentiation and contributes to the plant body. In this way, the population of meristem cells is continually renewed. Molecular genetic evidence supports the hypothesis that animal stem cells and plant meristem cells may share some common pathways of gene expression.