No. 612] GENESIS OF ORGANIZATION OF INSECT EGG 715 



ferments might constitute the factors responsible for the 

 modification of embryonic, larval and adult characters — 

 factors such as have been employed for experimental 

 breeding purposes by most geneticists. According to this 

 hypothesis it would probably be necessary to consider 

 the main portions of each chromosome as sufficient for the 

 production of an entire organism. The fact that the 

 group of factors carried by any one chromosome in Dro- 

 sophila controls characters that are not restricted to any 

 definite part of the body gives weight to this assumption. 



Most geneticists are accustomed to deal with adult char- 

 acters only, and on this account pay very little or no at- 

 tention to the eggs, embryos and larvae of the species they 

 are experimenting with. But the eggs, embryos and 

 larvae contain all the factors for these adult characters, 

 both those that are realized and those that are inhibited 

 either by internal or external causes, and they may ex- 

 hibit characters that make it possible to separate different 

 lines although the adults may be indistinguishable. Fur- 

 thermore, taxonomists have long recognized the value of 

 embryonic characters as an aid determining species. 



We should always be careful to distinguish between the 

 parts of the egg that are of hereditary significance and 

 those that are not. Thus the shell or chorion of the silk- 

 worm egg has been discussed under the heading of ''cyto- 

 plasmic inheritance," whereas it is not a vital part of the 

 egg, but, being secreted by the epithelium of the ovarian 

 tube, is a well-defined characteristic of the adult female 

 and its colorafion, which follows the laws of Mendelian in- 

 heritance,^^ is controlled by maternal factors. 



Such fundamental characteristics as polarity, s^-m- 

 metry, and pattern, which are so clearly exhibited by the 

 eggs of insects and certain other animals, are much more 

 difficult to study than adult characters and are probably 

 not so easily modified. If any or all of them are carried 

 over from one generation to another in the cytoplasm we 

 have then a real instance of cytoplasmic inheritance. 

 Even if this is the ease the chromosomes doubtless exert 



44 Toyama, 1913, Journ. of Genetics, Vol. 2. 



