362 Mendel's Experiments 



of both. It includes in three different classes four indi- 

 viduals. In the formation of these, pollen and egg cells 

 of the form A and a take part on the average equally in 

 the fertilisation ; hence each form [occurs] twice, since four 

 individuals are formed. There participate consequently in 

 the fertilisation 



The pollen cells A + A + a + a 

 The egg cells A+A+a 



It remains, therefore, purely a matter of chance which 

 of the two sorts of pollen will become united with each 

 separate egg cell. According, however, to the law of 

 probability 4 , it will always happen, on the average of many 

 cases, that each pollen form A and a will unite equally 

 often with each egg cell form A and a, consequently one 

 of the two pollen cells A in the fertilisation will meet with 

 the egg cell A and the other with an egg cell a, and so 

 likewise one pollen cell a will unite with an egg cell A y 

 and the other with egg cell a. 



Pollen cells A A a a 



\ x i 



Egg cells A A a a 



The result of the fertilisation may be made clear by 

 putting the signs for the conjoined egg and pollen cells in 

 the form of fractions, those for the pollen cells above and 

 those for the egg cells below the line. We then have 



4+-+4+-. 



A a A a 



In the first and fourth term the egg and pollen cells are of 

 like kind, consequently the product of their union must be 

 constant, viz. A and a ; in the second and third, on the 

 other hand, there again results a union of the two differen- 

 tiating characters of the stocks, consequently the forms 

 resulting from these fertilisations are identical with those 

 of the hybrid from which they sprang. There occurs 

 accordingly a repeated hybridisation. This explains the 

 striking fact that the hybrids are able to produce, besides 



