162 



The Nucleus and Cytoplasm in Development 



Among the latter is a class of asymmetries 

 which are not fixed in relation to the main 

 axis or secondary axes of the animal; thus 

 both right-left and left-right arrangements 

 occur in a group of affected individuals. For 

 instance, eye defects in the myelencephalic 

 strain of mice may be present on only one 



asymmetrical ones. Two types of symmetrical 

 individuals are found in each instance, those 

 with both sides normal, and those with both 

 sides affected. The problem of asymmetry is 

 thus bound up with that of variable expres- 

 sion of the genotype from one specimen to 

 the next. Basically the genes involved are 



Fig. 43. Homeotic mutants of Drosophila melanogaster. A, Frontal view of normal head; a, antenna with 

 arista {ar); pb, ventral section of proboscis (extended). B, Antenna of "aristapedia" (after Balkaschina, '29). 

 C, Two examples of proboscis of "proboscipedia" (after Bridges and Dobzhansky, '33). D, Bithorax; the 

 halteres are changed into wing-like structures. (Original, courtesy of E. M. Wallace.) 



side, but the two types "right eye normal, 

 left eye abnormal" and "right eye abnormal, 

 left eye normal" occur with equal frequency 

 (Bonnevie, '34). Similarly, in Drosophila, in 

 the Dichaete strain, there are often present 

 only three of the four normally occurring 

 thoracic dorsocentral macrochaetae, but the 

 two types "two right bristles, one left" and 

 "one right, two left" again are equally nu- 

 merous (Plunkett, '26). The developmental 

 interpretation of these asymmetries is easily 

 suggested by the occurrence of symmetrical 

 individuals of the same genotypes as the 



such that they do not always actually pro- 

 duce the phenotype by means of which their 

 presence can be recognized. Depending on 

 variations in the general genetic background 

 provided by the rest of the genotype, or on 

 variation in the external or internal environ- 

 ment of the developing system, the action of 

 a given gene may not always "penetrate to 

 the surface" of observable phenomena. In 

 the two examples given, the existence of 

 mice or flies which are normal on both sides 

 shows complete lack of penetrance of the po- 

 tentially abnormal genotype, while the ex- 



