360 PRINCIPLES OF EMBRYOLOGY 



to some factor in the cytoplasm. For instance, when conjugation occurs 

 between tAvo Paramecia, one carrying the antigen A and the other B, the 

 two individuals which separate again will be alike in their complements 

 of genes, but unless some interchange of cytoplasm has occurred, each 

 continues to produce its characteristic type of antigen. However, the 

 responsibility of the cytoplasm for the antigens is not absolute; the genes 

 determine the range of possible antigens which the Paramecium can 

 form. So far three main gene-loci have been studied, known as D, G and S. 

 Each strain possesses its characteristic alleles at each of these loci. Normally, 

 only one locus is active at a time. The important point for our present 

 discussion is that it is the condition of the cytoplasm which determines 

 which of the loci shall be in operation (Fig. 16.7). Exactly what is imphed 



Parents 



F^ animals 

 finally at 25" 



F^ animals 

 after 5 fissions 



Fi animals 

 finally at 29° 



(J»° (g«° s) 



■ (d'°) g^ (s) 





d<^ (g'o s) 



9\ d'" (g'° s) 



Figure 16.7 



The results of a cross between Paramecia of stocks 90 and 60. The former 

 have been kept at 25° C, and the state of their cytoplasm activates their 

 genes controlling G antigens (i.e.^^*^); while the latter have been at 29° C. 

 and D antigens are being produced. In the first few generations of the Fi, 

 the G or D states of the cytoplasm persist, and bring into action the corres- 

 ponding g or d genes in the hybrid : thus the individual with shaded cyto- 

 plasm now forms g^^ and g'''^ antigens, and that with unshaded cytoplasm 

 d^^ and d'^^. After some time, the cytoplasm of later generations adjusts itself 

 to the temperature at which the stocks are kept; and when it does so, acti- 

 vates the corresponding genes. (From Beale 1954.J 



