EVOLUTION, HEREDITY, EUGENICS 509 



divided up into cells which very early become arranged in two 

 layers, the outer ectoderm (epiblast) which covers the surface of 

 the embryo and the inner entoderm (hypoblast) which lines the 

 interior. Between the ectoderm and the entoderm, a third layer 

 of cells, the mesoderm (mesoblast), arises, usually coming directly 

 or indirectly from the entoderm. 



The ectoderm gives rise to the epidermis covering the body, the 

 hair and nails, and to organs derived from the epidermis-nervous 

 system including the retina, epithelial lining of the mouth and anus, 

 buccal glands and enamel of the teeth. 



The entoderm gives rise to the notochord and the epithelium 

 lining the alimentary canal and its diverticula-including glands of 

 the digestive tube, the lungs, bladder, hepatic cells of the liver and 

 secretory cells of the pancreas. 



The mesoderm gives rise to structures lying between the ectoderm 

 and the entoderm including all connective or supporting tissues 

 except neuroglia. From it originate striated and most smooth 

 muscle, blood vessels and lymphatics, peritoneum, skeleton and the 

 epithelium of the genito-urinary system except the urethra and 

 part of the bladder. 



Theories of Heredity and Evolution. — Hypotheses and theories 

 of heredity are legion and we shall mention only a few of the older 

 and less tenable ones before taking up the ones that are believed in 

 at the present time. The theory of Epigenesis supposed that 

 development came about through the agency of mysterious external 

 forces. The theory of Preformation supposed that the germ con- 

 tained all the parts that were to develop from it. We believe, how- 

 ever, that the union of the germinal elements gives rise to a new 

 organism that may have different qualities from the parents. We 

 recognize the fact that combinations may be produced by the 

 environment, and that apparently new types may be examples of 

 reversion and due to the ch7~omatin that has been handed down from 

 some remote ancestor. 



History of the Evolutionary Idea. — Many people have heard so 

 much of the Darwinian theory of Evolution without getting any 

 accurate first-hand knowledge of what Darwin said that they are 

 inclined to believe that Charles Darwin gave us the first idea of 

 evolution. This is far from the truth, however, since from the time 

 of the earliest Greeks traditions have been handed down regarding 

 the origin and development of man. 



