Role of Preformed Structure in Cell Heredity 167 



existing cell structure requires a cell that possesses conspicuous, con- 

 stant, normal organizational features. However, it also obviously re- 

 quires a cell with readily available or producible hereditary variations 

 of its normal features. Finally, the cell should be amenable to stand- 

 ard breeding analysis and other analytic procedures. Few, if any, 

 cell types possess the needed combination of qualifications to the de- 

 gree exhibited by the ciliated protozoan, Paramecium aurelia. Within 

 this species, stock 51 (of syngen 4) and its derivatives are especially 

 suitable because of the availability of adequate nuclear and cytoplas- 

 mic markers and an array of varied experimental tricks useful for 

 decisive experimental analysis. This material was therefore used. 



Attention was directed to the characteristic pattern of cortical or- 

 ganelles and its experimental modifications. The first and most ex- 

 haustively studied modification was the doublet cell, wbich possesses 

 two complete sets of cortical structures. Doublets and other cortical 

 variations in various ciliates, including Paramecium, have been studied 

 from a number of points of view by many workers (see page 198). 

 Some of their conclusions are closely similar to the ones reached in 

 the present study, the unique feature of which is the completeness of 

 the essential genetic analysis and some aspects of the search for gen- 

 eralities and mechanisms. Preliminary abstracts of some of the present 

 work have been published by Sonneborn and Dippell (1960a; 1961a, 

 b, c; 1962). 



If. The Pattern of Cortical Structure 

 in Paramecium aurelia 



A, The cortical pattern in normal singlets 



Most of the features of the cortical pattern of P. aurelia, which were 

 followed in this investigation, are shown in Fig. \A of the dorsal 

 ( aboral ) and Fig. IB of the ventral ( oral ) surfaces of a typical nor- 

 mal cell. The most conspicuous features of the dorsal surface are two 

 large dots on the same meridian or not more than a few meridians 

 apart. These are the pores of the contractile vacuoles. Normally, the 

 anterior pore is further from the anterior pole than the posterior pore 

 is from the posterior pole. The most conspicuous features of the ventral 

 surface are all on one meridian, the oral meridian. Near the middle of 

 this meridian is a large oval or comma-shaped depression, the vesti- 

 bule, at the base of which is the clear, open mouth (technically, the 

 buccal overture). Anterior to the mouth is a long, narrow, clear area 

 — the preoral suture — extending from the mouth forward and left to 



