136 THE CHROMOSOMES 



indefinitely. Such bodies might not only produce 

 passive products, like starch or pigment, but even 

 active enzymes, which, interacting with other 

 products of development, might determine the 

 characteristics of the race. 



Structures like the shell and the yolk of eggs are 

 purely maternal in origin, but since they do not have 

 the power of growth and division, they are not able 

 to perpetuate themselves indefinitely, nevertheless 

 they may determine certain characteristics of the 

 embryo, and to this extent may appear to influence 

 the hereditary characters of the generation to which 

 the embryo belongs. For instance, the females of 

 certain races of silkworm moths have white eggs, be- 

 cause the shell is white. If such eggs are fertilized by 

 sperm of another race, that has eggs with a domi- 

 nant green colored shell, the shells are nevertheless 

 white. Conversely when the green eggs of a female 

 moth of the green egg race are fertilized by the 

 sperm of a male of a white egg race, the color re- 

 mains green. When the moths develop from either 

 of these two kinds of hybrid eggs, one white, one 

 green, they lay only green eggs, because in the hybrid 

 the factor for green dominates and determines the 

 color of the shell that is produced in the new eggs. 

 These green eggs give rise to moths, three of which 

 lay eggs that are green to one that lays eggs that are 

 white, showing that here there is only the ordinary 

 case of Mendelian inheritance, which is obscured, 

 however, when the characters of the young embryo 

 are considered, because, as has been shown, these 



