THE BRIDGE OF LIFE 501 



sea water will undergo embryonic development. Rabbit eggs can be 

 treated with salt water, ice water, and pin pricks to stimulate the be- 

 ginning of such development. In one case, at least, one of these de- 

 veloping embryos was transplanted into the body of a virgin female 

 rabbit and it developed and was born as a normal-appearing female 

 (Fig. 31.8). 



These experiments show that the sperm does not bring some sub- 

 stance into the egg which stimulates the changes leading to the cleavages. 

 Rather it would appear that the egg has within itself a reaction system 

 which is all ready to operate when the proper stimulus is applied. 

 The sperm normally applies this stimulus, but experiments show that 

 it can be applied in other ways. It may be that there is some enzyme 

 within the egg which is held in a bound form ; but, when released by 

 the presence of the sperm or by experimental means, this enzyme brings 

 about the changes which are associated with early embryonic develop- 

 ment. 



Genes and Ancestry 



It will be noted from the events described in this chapter that an 

 individual receives only one half of the genes possessed by each parent. 

 The other half are irretrievably lost. A person can never express those 

 lost genes in himself or in any of his descendants unless they are brought 

 in by other persons through marriage. Also, since each parent received 

 half of his genes from his parents, it is evident that every person re- 

 ceives approximately one fourth of the genes possessed by each grand- 

 parent, an eighth from each great grandparent, and so on. You may 

 have had an ancestor that came over in the "Mayflower," but the pro- 

 portion of this ancestor's genes in your cells is quite small in comparison 

 to those received from the hundreds of other ancestors of the same 

 period who transmitted their genes to you. Each of these others con- 

 tributed just as much even through he or she may have suffered a 

 witch's death in Salem, ended life on a hangman's noose, or finished life 

 in an insane asylum. Genes are no respectors of persons. 



REVIEW QUESTIONS 



1. A human egg is thousands of times larger than a human sperm, yet chil- 

 dren tend to inherit equally from each parent. Explain. 



2. Why did the theories of heredity before the seventeenth century fail to take 

 account of the role of sperms and eggs ? 



3. How did Weismann's germ plasm theory of inheritance differ from Darwin's 

 theory of pangenesis ? 



4. What was the contribution of Mendel to an understanding of inheritance? 



