CONTINUITY OF THE RACE 



611 



surrounded by an environment. Very soon, 

 as cleavage starts, tlie surrounding environ- 

 ment (E) contributes to the cytoplasm, 

 thereby changing it slightly. Moreover, the 

 immediate community of cells has its effect 

 upon the cytoplasm of each of the other 

 cells. The cytoplasm is therefore influenced 

 from within by the genes (G) and from 

 without by the environment (E), even at 

 tliis early stage. As development proceeds, 

 the cytoplasm of the many different cells 

 continues to be influenced by surrounding 

 cells (C), yet the genie constitution re- 

 mains essentially the same in all of them. 

 All of this brings about the development 

 of organ systems, and ultimately the full- 

 fledged fetus, but in each step the genes 

 control the cytoplasm which, in turn, is 

 influenced by surrounding cells (organiz- 

 ers) so that the cells are channelled into a 

 pattern directed toward the final organized 

 individual. Another environmental factor 

 that is influential is the nourishment (N) 

 received by the fetus from the uterine wall. 

 If food, as well as oxygen and hormones, 

 that come to it from the placenta are de- 

 ficient in any respect, the fetus is directly 

 affected, perhaps to a point where normal 

 development is impossible. This is a very 

 important environmental aspect of develop- 

 ment. The genes strive toward the produc- 

 tion of a normal offpsring but they can 

 accomplish this end only if the environment 

 is adequate. 



When the child is born, another factor, 

 the psychological factor (P), enters the pic- 

 ture. All of the other factors still operate, 

 and the addition of this new factor defi- 

 nitely controls further mental development 

 and probably physical development as well. 

 Lastly, as the chfld matures, the social fac- 

 tors ( S ) become more and more a dominant 

 part of his environment. The genes are still 

 at work and will continue to be throughout 

 life but the environment has a marked 

 molding effect on their action. Thus, we 

 see that it is impossible to have heredity 

 without environment and vice versa. The 



two forces work hand in hand to produce 

 a normal, well-adjusted individual. 



The occasional appearance of two off- 

 spring as a result of one fertilized egg 

 (identical or monozygotic twins) has pro- 

 vided almost perfect material for the study 

 of the relative effects of environment and 



fro^'crndl 



Fig. 24-18. Identical or monozygotic twins are a result of 

 a single fertilized egg, therefore they are genetically 

 alike as shown in the upper figure. Fraternal twins 

 are a result of two zygotes and may be as genetically 

 different as any other members of the family. Further- 

 more, each developing fetus has its own placenta, 

 whereas this is usually not the case with monozygotic 

 twins. 



heredity (Fig. 24-18). Monozygotic twins 

 have the same sets of genes, so that if 

 heredity were solely responsible for all of 

 theii- characters they should be exactly 

 alike. A study of a large number of identical 

 twins by Newman and others has shown 

 that they are indeed remarkably, almost 

 unbelievably, alike (Fig. 24-19). Such traits 

 as height, weight, general appearance, fin- 

 gerprints, and tooth formation show very 



