Membrane bones develop (or ossify) when certain cells (osteoblasts) become bone secreting. The osteoblasts with numerous fibres of connective tissue cells, form an organic network. Layers of calcium salts (lamellae) are deposited into this network, and slowly a plate is built up. The plate can grow about its margins and also can be thickened on either its inner or outer surface. Some of the osteoblasts remain alive and actually become incorporated into the growing bone. They are then known as osteocytes and come to have very irregular shapes. The spaces (lacunae) in which the ostecocytes are situsted are also irregular with long branching processes (canaliculi) extending in all directions. The canaliculi of neighbouring spaces link up and provide a network through which blood containing food and oxygen can filter.

Cartilage bones are not formed directly; they are replacement structures. In the developing embryo most of the skeletal structure is first laid down as cartilage. This softer material is only gradually replaced by bone a process which is not completed until the animal is full grown. In Man the ossification of the cartilage continues until the age of 25 or 26.

In shape the embryonic cartilage structures are miniatures of the final adult bones. They begin their transformation into bone soon after formation. First, the cartilage becomes calcified (impregnated with calcium salt) and many of its cells die. Then large amoeboid cells called osteoclasts penetrate into the calcified cartilage and begin to destroy it. They soon create a series of channels and blood vessels break in. With the blood come osteoblast cells and bone begins to be laid down.

The cartilage of many vertebrates ossifies from one centre only. For instance the limb bones start to ossify in the middle and the process continues towards either end of the structure. The zone of ossification is called the diaphysis. In mammals, in addition to the diaphysis, other points of ossification may be set up. Each accessory point is called an epiphysis. Epiphyses are especially to be found on bony projections for muscle attachment or places which articulate with other bones. Thus, though ossification is incomplete as a whole, those parts which must particularly stand up to strain are already reinforced.

If the original cartilage did not grow, hone would soon replace all of it. But growth does continue. In the limb bones for instance as fast as ossification takes place towards their middles, so new cartilage is formed towards the extremities. Only when the adult skeleton is completely grown does cartilager stop developing; ossification is at last able to catch up.

The advantage of cartilage as an early skeletal tissue is that though less rigid than bone it can increase its volume from the inside. Bone, on the other hand, grows only by additions at its surface. Parts of the bone surface are attached to muscles or are delicately articulated with neighbouring bones. Additional bone forming at the surface would disrupt these connections.

But nevertheless the larger cartilage hones making the limbs are not entirely formed by replacement of cartilage. When ossification begins at early stage, the cartilage structures are still very small in diameter. But later, the cartilage forming towards the extremities will be expanded in width. Bone replacing the cartilage produces a shaft that tapers towards the middle.

A bone shaped like an hour glass would be very weak at its middle. The problem is solved by additions of bone to parts of the cartilage surface. The bone is particularly thick in the original, thin middle part of the structure. This coating of bone is, strictly speaking, membrane bone for it does not replace earlier cartilage. It forms on top of the cartilage and for this reason is called perichondral bone.

Throughout the life of an animal, bone is continuously being reworked. Old material is resorped by the osteoclasts and new material deposited by osleoblasts. Bones which have been remodelled can be recognized in thin section under the microscope by the development of Haversian canals. The canals which usually run parallel to one another, carry blood vessels and nerves. The blood vessels carrying food, pass out into the surrounding bone. Osteoblasts become arranged in a series of concentric circles about each canal They rely upon the blood vessels in the canal for a supply of nourishment. Each canal with its dependent series of osteoblasts constitutes an Haveversiany system.

Bone varies in density, strength and hardness. The outer portions of bony structures usually consist of compact bone in which the percentage of living cells and organic substances is small. The inner surface of the larger bones is made of spongy bone. This consists of just a framework of bony bars giving rigidity; in between are fatty, vascular, and other tissues forming part of the bone marrow. Spongy bone is light and lowers the overall weight of the skeleton without losing much of its strength. The very centre of the larger bones may become hollow. Replacing the bone are the son; fatty tissues of the bone marrow where fats are stored and blood corpuscles made.

Bowman's Capsule - A chamber in the vertebrate kidney, one connected to each renal tubule. Inside is the glomerulus a knot of blood vessels where waste is removed from the blood. The capsule and glomerulus make up the Malpighian body.

Beachial - Concerning the arms.

Beachiation - Method of movement, practised especially by apes, involving swinging from hand to hand in the trees.

Brachiopoda - A phylum of animals with a very long fossil history but relatively few living forms. They are shelled animals, the shell having two valves which may or may not be higed. The characteristic feature is a pair of coiled ciliated arms which are used for feeding. The cilia draw a current of water towards the mouth which collects any food particies from the water. Brachiopods are commonly called lamp shells.

Brain - The co ordinating centre of the nervous system. It is the front part of the central nervous system andas well is its general co ordinating function it is specially concerned with the sense organs of the head and the special senses. Coelenterates, starfishes, and other animals without well defined heads do not have the concentration of nervous tissue called the brain but all animals that are bilaterally symmetrical have some sort of brain even if it is merely a slight swelling of the nerves in that area.

The brain is most highly developed in cephlopods and in vertebrates. Three main regions can be detected in the primitive vertebrate brain: the fore brain, consisting mainly of the olfactory lobes; the mid brain, made up largely of the optic lobes controlling the sense of sight; and the hind brain which is composed of the cerebellum and the medulla oblongata. The cerebellum is concerned largely with balance and movement while the medulla is the centre for the senses of taste and hearing among other things.

These three regions can be detected in all vertebrate brains but their relative sizes differ a great deal according to the habits of the animals. Sharks rely largely on their sense of smell and the olfactory lobes are correspondingly large. Most bony fishes rely mainly on sight and so the optic region is large. The basic pattern is similar in amphibians and reptiles but there is a marked change in the fore brain; it is roofed over and developed as the cerebral hemispheres. These receive nerve tracts from other parts of the brain and nervous system. The fore-brain is developing into the main association 'centre' controlling the actions of the animals.

This development of the cerebral hemispheres is carried further in the birds and mammals and they come to dominate the whole brain. The roof of the cerebral hemispheres is known as the cerebral cortex and in man it spreads back so as to almost cover the rest of the brain. The cortex is made up of millions of cells known as 'grey matter' and overlies the 'white matter' made up of the nerve axons that conduct messages to the from other parts of the brain. The cortex controls the behaviour of the animals and it has been found that each region of the cortex controls certain aspects. In may for example, there are definite regions for speech, memory, sight, etc.

Branchial - Concerning the gills e.g. branchial arteries.

Branchiopoda - Sub class of the Crustacea including the fairy shrimp and the water fleas

Bristle-worm - (See Polychaeta).

Brittle-star - (See Echinodermata).

Bronchus (plural bronchi) - The vertebrate trachea divides into two bronchi, one leading to each lung.

Bronchiole - One of the branches of a bronchus within the sung.

Bryozoa - (=Polyzoa) Sea mats, a group of largely marine animals often found encrusting rocks and seaweeds. Now split into two phyla; Endoprocta and Ectoprocta.

Buccal Cavity - The mouth cavity which in vertebrates contains the teeth and tongue.

Bug - Popularly used for any insect, this term should strictly be applied only to members of the order Hemipfera.

Butterfly - (See Lepidoptera).