390 



PRINCIPLES OF EMBRYOLOGY 



some strains, known as 'killers', produce and secrete into the medium a 

 substance which kills certain other strains, known as 'sensitives'. The pro- 

 duction of the killer substance is controlled by cytoplasmic particles known 

 as kappa, and these in turn depend on the nuclear alleles K and k. The 

 importance of the cytoplasmic particles is demonstrated by the experi- 

 ment summarised in Fig. 18.3 ; if KK killers are crossed with kk sensitives, 

 all the offspring will have the genie constitution Kk, but only those wliich 



Figure 18.2 



Types of plasmagene. The diagram shows a cell containing a nucleus, and 

 the following types of plasmagene : E, exogenous, originating from outside 

 the cell; can usually multiply only with the co-operation of the nucleus, 

 which may also influence its physiological effect a; P, a true plasmagene, 

 independent of the nucleus both in multiplication and in effect h\V,z visible 

 cytoplasmic particle; can usually multiply independently of nucleus, but 

 may affect the latter (cf. Stentor), while die nucleus may influence its physio- 

 logical effect c; G, a gene-initiated plasmagene, originating under the in- 

 fluence of the nucleus, but multiplying and being physiologically active in 

 relative, though not complete, independence of it. 



derive their cytoplasm from the killer parent will contain kappa and 

 behave as killers. If, in such a cross, the period of union of the two Pro- 

 tozoa is unduly prolonged, some kappa-containing cytoplasm may pass 

 into the offspring derived mainly from the sensitive parent, and provided 

 these offspring contain at least one K gene, the kappa particles multiply 

 (Fig. 18.4). On the other hand, in cells containing only the recessive ky 

 kappa particles which may originally be present fail to multiply and gradu- 

 ally become diluted out of existence as the strain of Protozoa proliferates. 



