32 HOW ANIMALS DEVELOP 



Fertilization^ Heredity^ Virgin Birth 



At the end of all this preparation the germ-cells are 

 ready to carry out the complicated process of 

 developing into an adult, and are finally ready for 

 fertilization. Fertilization really consists of two 

 processes, the activation of the ^gg by the sperm 

 and the union of the o^gg and sperm nuclei. It is 

 easy to see the importance of the second process ; it 

 restores the diploid number of the chromosomes by 

 adding the haploid number in the sperm to the 

 haploid number in the ^gg. A properly balanced set 

 of chromosomes is essential for the development of 

 the animal since they contain the hereditary factors. 

 An example will show how the influence of the 

 hereditary factors can be detected. Men are some- 

 times born with short fingers, each with only two 

 joints instead of three, because of some abnormality 

 in the development of the fingers. The character is 

 hereditary. For instance, we find short-fingered men 

 who have married normal wives and all of whose 

 children have short fingers. If two children born of 

 such parents then marry, on an average one-quarter 

 of their children will be normal and three-quarters 

 short-fingered. These facts are due to the presence 

 of a hereditary factor or gene for short fingers lying 

 in a chromosome. The short-finger gene is an 

 abnormal form of the gene which causes the fingers 

 to develop in the ordinary way, and is derived from 

 it by a sudden and as yet inexplicable change called 

 a mutation. The original short-fingered fathers have 



