TYPES OF MENDELIAN HEREDITY 43. 
with reduplicated legs, where a factor difference has a 
visible effect only under special external circum- 
stances. 
A number of cases of Mendelian inheritance are 
known in which only the larve, and not the adults, 
are affected. Tower has described crosses in which 
the beetle Leptinotarsa signaticollis was crossed 
with L. undecimlineata (Fig. 20, 4, B). In the first 
stage (C), the larve of these two beetles are exactly 
alike, but in the second stage, the larve of L. undecim- 
lineata are white and the larve of L. signaticollis are 
yellow; and in the third stage the undecimlineata larvee 
are still white without stripes, while the others have 
well-developed tergal stripes (B). When these species 
are crossed under certain external conditions the F, 
larve are yellow and, later, striped. The beetles that 
come from them are intermediate. Inbred, these 
beetles give three larve 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 peculi- 
arities of different parts of the egg and embryo, may, 
