MENDEL^S EXPLANATION 175 



Tlie most convenient way of representing the 

 supposed causes of Mendelian segregation on paper 

 is to use the four-square table. Along the top of 

 the table are written the two kinds of factors that 

 occur in equal numbers amongst the male germ cells 

 of the hybrid, namely, the tall, for which the letter 

 T is written, and the dwarf, for which D is written. 

 The same is written along the left of the table, but 

 the T and D here refer to the factors in the female 

 germ cells. 



c? 



T 



D 



T 



D 



Two squares alongside one another are called 

 a row. Two squares superimposed on one another 

 are called a column. The T above the whole square 

 relates to the column below it, and the D, to the right, 

 to the column below it. Similarly the T and D at 

 the left of the table relate to the rows to the right 

 of them. In the square formed by the intersection 

 of a row with a column is written the result of the 

 union of the factor at the head of the column with 

 the factor at the left of the row. This Table is 

 merely another way of stating what has already been 



