500 



THE BRIDGE OF LIFE 



through the cell membrane and would be correlated with the higher 

 rate of metabolism within the cell. Careful studies also show that the 

 protoplasm of the egg increases in viscosity — it becomes more solid — 

 along with these other changes and this is apparently necessary for cell 

 duplication. Without these vital changes in the eggs there would be 

 no embryo even though the sperm head should enter with its load of 

 chromosomes. 



It is possible to achieve an artificial stimulation of the egg which 

 causes it to undergo the physiological reactions which normally come 

 only with fertilization and in some cases an embryo may even develop. 



SHBIHH&fflGS 



Courtesy of Gregory Pincus 



Fig. 31.8. A rabbit without a father. The little female bunny on the left, shown with 

 its mother, was produced from an unfertilized egg. The egg was artifically induced 

 to begin division without the entrance of a sperm. This shows that the egg contains 

 all the genes necessary for the production of an individual, but normally will not do 



so without union with a sperm. 



We have already learned that within the egg of most forms of animal 

 life there are sufficient chromosomes and genes to form a complete 

 individual. Normally, there will be two of each kind of chromosome 

 in the body cells, yet one of each kind is all that is absolutely necessary. 

 In some animals the stimulus of fertilization is not necessary — we 

 learned in Chapter 16 that the infertile eggs of queen bees go ahead and 

 form embryos which becomes drones. Thus the drone bees carry only 

 one set of chromosomes and one set of genes in their body cells, while 

 the female bees have the more common double set because they come 

 from fertilized eggs. A similar development of unfertilized eggs has 

 been accomplished experimentally in other animals. If a frog's egg is 

 pricked with a needle covered with blood serum it will begin embryonic 

 development and in some cases will develop into a mature frog. Sea 

 urchin eggs placed in strong salt solution and then returned to normal 



