34 2 ANATOMY OF THE DOMESTIC FOWL 



museum specimens each of which represents the development of a 

 day up to and including the twenty-first day. 



THE DERIVATIVES OF THE GERM-LAYERS 



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 epi- 

 thelium 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 communicat- 

 ing 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, 

 Jymph, 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. 



