Part 95

From the three primary germ-layers are developed the various tissues and organs of the body by metamorphoses which may be referred to the two fundamental processes of specialization, or the adaptation of structure to function, and of unequal growth, which latter results in the formation of folds, ridges, and constrictions.

From the ectoderm are produced:

The epidermis and its appendages, including the nails, the epithelium in connection to the feathers and the feathers.

The infoldings of the epidermis, including the epithelium of the mouth, epithelium of the salivary glands and the anterior lobe of the pituitary body, or hypophysis.

The epithelium of the nasal tract with its glands and communicating cavities.

The epithelium lining the external auditory canal, including the outer stratum of the membrana tympani.

The lining of the anus.

The epithelium of the conjunctiva and of the anterior part of the cornea, the crystalline lens.

The spinal cord, the brain with its outgrowths, including the optic nerve, the retina, and the posterior lobe of the pituitary body.

The epithelium of the inner ear.

From the entoderm are produced:

The epithelium of the respiratory tract.

The epithelium of the digestive tract, from the back part of the pharynx to the anus, including its associated glands, the liver, and the pancreas.

The epithelial parts of the middle ear and of the eustachian tube.

The epithelium of the thymus and the thyroid bodies.

From the mesoderm are developed:

Connective tissue in all its modified forms, such as bone, cartilage, lymph, blood, fibrous and areolar tissue.

Muscle tissue.

All endothelial cells, as of joint-cavities, bursal sacs, lymph sacs, blood-vessels, pericardium, and endocardium, pleura, and peritoneum.

The spleen.

The kidneys and ureters.

The testicles and the system of excretory ducts.

The ovary and oviduct.

PREPARATION OF STRUCTURES FOR STUDY

It is hoped that the following suggestions will be helpful in the laboratory work.

=Directions for Dissecting Muscles.=—The muscles that are brought into great play in movements of the bird’s limbs are dark carmine in color, while those which are not brought greatly into use are pale or white in color.

The tendons are made up of white fibrous connective tissue, are very dense, and pearly white in color.

In securing a bird for dissection of muscles it is best to select one in rather poor flesh, as the fat is annoying. The bird may be chloroformed or killed in a bell jar by aid of illuminating gas. After the bird is dead pluck all the feathers and immerse it in a 10 per cent. solution of formaldehyde or of 80 per cent. alcohol. It is best to puncture the abdominal wall so that the fluid may at once fill the abdominal cavity and more readily gain access to the chest cavity; and to puncture the skin at various points so that the liquid may more quickly become disseminated among the muscular structures. Post-mortem changes quickly take place if these precautions are not taken. The liquid surrounding the carcass should be at least twice the quantity of the bulk of the carcass.