712 General and Applied Biology 



The outer layer of cells of the gastrula is known as the ectoderm layer; 

 the inner layer, the entoderm layer; and a middle layer, formed by both 

 of the other layers, the mesoderm layer. 



All these cells have received a portion of the original inherited mate- 

 rials which they will continue to pass on to future cells as they are formed. 

 Thus, we see how an organism retains what it inherited at fertilization. 



Various types of tissues arise from the three layers described above. 

 Naturally, at some stage there will be set aside the first cell, or primordial 

 germ cell, which will later develop into sex organs which in turn will 

 produce sex cells. Thus, the cycle is completed. The reproductive or- 

 gans arise from the mesoderm layer. 



This cycle shows the continuity of life and the immortality of the germ 

 plasm. The somatoplasm or body plasm which develops from the germ 

 plasm is temporary and is mortal or dies. 



18. Inheritance or Noninheritance of Acquired Characters. — Acquired 

 characters are those responses or structural modifications acquired by an 

 organism in its attempt to adjust itself to the various factors of the en- 

 vironment which surround it. Because most of the acquired characters 

 affect only the body plasm (somatoplasm), it is considered that they are 

 usually not inheritable, although some experimental evidence seems to 

 point to the opposite view, at least in a few instances. Probably the cor- 

 rect conclusion is that most acquired characters are not inherited while 

 a few may be. This does not mean that environment has no effect, be- 

 cause it is known that the development of even inherited characters of 

 any individual organism depends upon the proper environment in which 

 the inherited genes may develop properly. In other words, both nature 

 and nurture are necessary for development. 



19. Human Inheritance. — 



Why Study Human Inheritance? A study of human heredity gives 

 us an insight into the question of how and why we, as individuals, have 

 come to be what we are and how and why we act as we do, through the 

 interaction of various external and internal environmental factors and 

 our inherited materials. We also may observe that human inheritance 

 follows the same laws of heredity that pertain to other organisms. We 

 may become familiar with the methods of race improvement; with the 

 main applications of heredity and environment in our dealings with 

 various sociologic, educational, and legal problems of our daily life. 

 Through a knowledge of human heredity, we may have a more sympa- 



