184 The Nature of Biological Diversity 



structures of the rest of the body were single: there was a single dorsal 

 surface with the usual solitary narrow segment bearing contractile 

 vacuoles. These features of the new cell type were as a rule faithfully 

 reproduced during successive fissions. 



This new hereditary cell type is theoretically decisive. A piece of 

 paroral cortex, pulled off of one cell and incorporated on the surface 

 of another cell, has led to the development and inheritance of an 

 entire supernumerary oral segment from pole to pole. In other words, 

 genetic autonomy of a delimited part of the cell cortex has been 

 demonstrated by the consequences of a natural graft of a small piece 

 of cortex. 



Further, observations on the cytopyge in the new cell type gave a 

 clue to its developmental and genetic determination. Unlike the nor- 

 mal situation, the third cytopyge is not located on a postoral suture; 

 but like the normal situation, it is located at the juncture of right and 

 left postoral kinety fields. A third juncture of this sort exists in these 

 cells as a result of the positions of the two oral segments: the left 

 postoral field of the right oral segment abuts on the right postoral field 

 of the left oral segment. And there the third cytopyge appears in 

 spite of the absence of a postoral suture and in spite of the reversal 

 of right-left relations. The development and position of the cytopyge 

 are thus correlated with localized structural features, the border be- 

 tween two specific different cortical fields. 



The second example of "cortical picking" was the reciprocal of the 

 one just described. The doublet conjugant robbed its singlet mate of 

 a paroral piece of cortex. The singlet failed to grow and died. The 

 doublet developed a third oral segment close to one of its two pre- 

 existing segments. This too was inherited, but reversion to the doublet 

 condition occurred with considerable frequency. 



The preceding examples indicate that an interpolar segment of cor- 

 tex, when integrated into a cell, behaves with a high degree of genetic 

 autonomy. Because growth of Paramecium is in the longitudinal di- 

 rection, the question arises as to whether genetic autonomy is pos- 

 sessed only by one or more entire meridians from pole to pole. The 

 answer to this question is provided by a type of clone which has arisen 

 independently several times and in more than one way. Clones of this 

 type show that some parts of an interpolar segment can be inherited 

 while other parts of the same segment are lacking. In other words, 

 genetic autonomy is not restricted to entire longitudinal segments, 

 but holds also for parts of a segment along the anteroposterior axis. 



Animals of this type (Fig. 9), have two oral segments 180° apart, 

 but only one of them is complete. The other oral segment lacks vesti- 



