74 HISTORY OF THE HUMAN BODY 



condition furnishes the best possible starting point from which 

 to follow its gradual modifications step by step until the adult 

 form is reached. The derivation and original anlage of most 

 of the systems have been given above and are expressed graph- 

 ically in several of the diagrams [Plates I and II; Fig. 14], 

 but it may be also useful to introduce the chapters on the 

 several systems by a table which shows the derivation of each. 

 In studying this it must be borne in mind that the mesenchyme, 

 which is everywhere distributed and forms all of the connect- 

 ive tissues of the body, enters into the final structure of every 

 other part, and hence is not taken into consideration here. 



EMBRYONAL ELEMENT. DERIVATIVE 



I. Ectoderm Epidermis; including that of the entire exter- 

 nal surface, as well as the more external 

 parts of mouth cavity, rectum, and other 

 cavities opening to the exterior. 

 Epidermic structures; including all glands of 

 the integument, nails and claws, hair and 

 feathers, horny scales, the enamel of the 

 teeth and the crystalline lens. 

 Nervous System; including brain and cord; 

 peripheral nerves and sympathetic system 

 with the ganglia associated with each; the 

 epithelium of the sense organs, and the 

 tapetum of the eye. 



II. Endoderm Alimentary canal, that is, its essential layer, 



the mucous membrane; also all organs de- 

 rived from this, as thymus and thyroid 

 glands, larynx, trachea and lungs, liver and 

 pancreas. 



Notochord; the anlage about which the ver- 

 tebrae (mesenchymatous structures) are 

 formed. 

 III. Mesoderm. 



a. Epimeres. .. .Voluntary muscles, except those of jaw, hyoid 



and branchial arches. 



b. Mesomeres. . Urogenital system, including the germ glands. 



c. Hypomeres. . Peritoneum; including pleura of mammals; 



germ-glands; voluntary muscles of jaw, 



