194 THE BIOLOGY OF STENTOR 



Conditions of embryological induction seem to be fulfilled: a 

 fine-stripe area remains morphologically inert as ventral ectoplasm 

 until it is brought into association with a wide-stripe area. The 

 latter itself never produces further elaborations, but in its associa- 

 tion with a fine-stripe area there are produced the oral structures 

 of Stentor. 



Whatever the difficulties to be resolved as we learn what actually 

 happens at the primordium site, the concept of the l.s.c. is a useful 

 guide. It explains why a graft complex produces more than one 

 primordium — because it has more than one primordium site or 

 l.s.c. — and this is not explainable in terms of either cytoplasmic 

 or nuclear volumes. It explains why any nucleated fragment cut 

 from locations far from the normal primordium site can neverthe- 

 less regenerate: because of the graded stripe widths around the 

 cell, it is almost impossible to produce a piece which on healing 

 will not bring stripes into juxtaposition with other stripes that are 

 not so wide and hence produce a sufficient anisotropy to occasion 

 primordium formation. 



It is therefore remarkable that on the cell level in Stentor we 

 find something very much like induction as manifested in the 

 embryogenesis of amphibia. In both cases there is the evocation 

 of a major elaboration determining the principal axis of the 

 organism — neural tube in salamander and feeding organelles in 

 Stentor — around which a new individuality can be organized (see 

 Fig. 55d). This evocation in both can be brought about by the 

 juxtaposition of certain parts, and is followed by a regionalization 

 or secondary induction, which in amphibia determines which end 

 of the tube will form the brain and in stentors is represented by 

 mouthparts formation under the influence of the posterior end of 

 the cell. What the significance of this striking parallel may be we 

 cannot yet say, but the consequences of the fact that induction 

 need not be intercellular could be of considerable theoretical 

 importance. 



