TYPES OF MENDELIAN HEREDITY 43 



with reduplicated legs, where a factor difference has a 

 vdsible effect only under special external circum- 

 stances. 



A number of cases of Mendelian inheritance are 

 known in which only the larvae, and not the adults, 

 are affected. Tower has described crosses in which 

 the beetle Leptinotarsa signaticoUis was crossed 

 with L. undecimlineata (Fig. 20, A, B). In the first 

 stage (C), the larvae of these two beetles are exactly 

 alike, but in the second stage, the larvae of L. undecim- 

 lineata are white and the larvae of L. signaticoUis are 

 yellow ; and in the third stage the undecimlineata larvae 

 are still white without stripes, while the others have 

 well-developed tergal stripes {B) . When these species 

 are crossed under certain external conditions the Fi 

 larvae are yellow and, later, striped. The beetles that 

 come from them are intermediate. Inbred, these 

 beetles give three larvae of the yellow type to one of 

 the white type. 



There is extensive evidence from cytology, experi- 

 mental embryology, and regeneration, to show that 

 all the different cells of the body receive the same 

 hereditary factors. We must suppose, then, that 

 the Mendelian factors are not sorted out, each to its 

 appropriate cell, so that factors for color go only to 

 pigment cells, factors for wing-shape to cells of the 

 wings, etc., but that differentiation is due to the cumu- 

 lative effect of regional differences in the egg and 

 embryo, reacting with a complex factorial background 

 that is the same in every cell. These regional pecuh- 

 arities of different parts of the egg and embryo, may, 



