228 



PHYSICAL BASIS OF HEEEDITY 



In certain races of tlie domesticated silkworm moth, 

 Toyama has shown that pigment develops in the em- 

 bryonic membrane (serosa) which, seen through the 

 egg-shell, gives a specific color character to the embryo. 

 It is not clear from Toyama 's account whether the pig- 

 ment is present at first, scattered in the cytoplasm, and 

 collects later at the surface, or whether it develops only 

 after the embryo develops. "When races are crossed with 

 characteristic but different embryonic membranes, the 

 color of the hybrid is like that of the maternal race only. 



Px 



Fi 



Fs 



' $D by c? rO 



O^R by c? D® 



Eggs and embryo . 



Genetic 

 constitution. 



Eggs and embryo . 



Genetic consti- 

 tution of F2 

 individuals . 



DD 



D R 



DR 



Ps EggB 



9 • 



1 2 



RR 



O 

 R 



1 



DD 



O 

 D R 



D 

 ® 



DR 



RH 



m o 



DDR 

 12 1 



Fig. 105. — Diagram to illustrate maternal inheritance. The black circle stands for a dom- 

 inant character affecting the serosa coat of the embryo. 



If adults {Fi) are raised from these eggs, it is found 

 when they in turn produce embryos, that the color of their 

 embryonic membrane is determined by the dominant char- 

 acter of the preceding generation that had been carried 

 in the chromosomes, irrespective of whether it came in 

 from the father or the mother (Fig. 105) . That the result 

 is really chromosomal is shown by still another generation 

 in which some of the females show the dominant character 

 in the membranes of their embryos and others no color 

 in the ratio of 3:1. 



It appears therefore in this case, the only one known 

 that furnishes critical evidence, that maternal inheritance 



