76 The Thied and Fourth Generation 



although in many cases we as yet have no 

 visible demonstration that such is the case. 



If we letter the chromosomes in the ancestors 

 of an individual so we can keep track, generation 

 after generation, of certain ones, we might have 

 some such scheme as the following to represent 

 the source of the chromosomes in the individual. 

 For the sake of simplicity it is assumed that in 

 the particular animal considered there are only 

 two chromosomes in the body cells. The 

 squares and large circles represent the individ- 

 ual animals, male and female, respectively; 

 eggs and sperm are represented by conventional 

 smaU circles and tailed figures. The cells of 

 the individual animal, one of which is repre- 

 sented in the lower row of the diagram, were 

 derived from the fertilized egg by its repeated 

 cleavage. This cell, as all of its fellows, contains 

 two chromosomes designated b and o; b comes 

 from the egg, o from the sperm, which of course 

 were derived from the mother and father, 

 respectively. Similarly, the mother's cells with 

 chromosomes b and / were derived from a 

 fertilized egg, the unfertilized egg contributing 



