198 Cytoplasm as Seat of Genetic Properties 



due to the chemistry of the product involved. Such is the case in the 

 work of Caspari (1933) on a pigment-reducing mutant in Ephestia, 

 which acts during the whole larval period, though it starts in the 

 unfertilized egg; the reason is that the mutant affects the production 

 of kynurenine needed for pigment synthesis. Some such variants which 

 have to do with transformation of kynurenine into 3-hydroxykynu- 

 renine in Bombyx have been found by Kikkawa (1953); the mutant, 

 which shows maternal inheritance, prevents the precursor substance 

 from diffusing into the egg. But in a number of cases of maternal 

 inheritance based upon a mutant locus within a chromosome acting 

 upon the egg before fertilization, the action affects a generalized 

 plasmatic feature of the egg (e.g., handedness of spindle), expressed 

 not simply as a Mendelizing trait of the egg but as a general predeter- 

 mination or conditioning of the cytoplasm, which may show its effects 

 throughout the development of the organism (e.g., the asymmetry 

 in a mollusk). Though controlled by a mutant locus primarily, and 

 thus being in the nature of an ordinary Mendelian trait, the conse- 

 quent conditioning of the cytoplasm of the egg is of general conse- 

 quence for development and therefore may be described as a kind of 

 intermediate condition between nuclear and cytoplasmic heredity, or 

 more correctly as a model for the way in which the simplest, most 

 generalized type of cytoplasmic heredity works. In view of this, a few 

 of the diflFerent types of maternal heredity may be discussed. 



The least generalized of these and also the best analyzed, because 

 both genetics and cytology are known, is the maternal efiFect in the 

 production of mosaics in the silkworm (Goldschmidt and Katsuki, 

 1927, I928a,b). A recessive mutant conditions the cytoplasm at the 

 time of the maturation divisions so that it prevents the normal linear 

 arrangement of the four resulting nuclei, of which only the proximal 

 nucleus is fertilized normally. Instead, the second polar nucleus 

 moves to a position parallel with that of the first, the egg nucleus, so 

 that now two egg nuclei are in correct position for fertilization and 

 are both fertilized, giving rise to somatic as well as sex mosaics in the 

 proper genetic and chromosomal combinations. 



Here the plasmatic conditioning is visible only in the movements 

 of the reduced nuclei. In the well-known cases of sinistrality in snails 

 (Boycott et at, 1930), previously mentioned, the mutant locus affects 

 the molecular orientation of the cytoplasm, resulting visibly in counter- 

 clockwise torsion of the spindles and subsequent sinistral spiral 

 cleavage. Thus the conditioning of the cytoplasm on a molecular level 

 controls a major feature of subsequent development. Here we see 



