Role of Preformed Structure in Cell Heredity 205 



the juncture or within the fine stripe zone. The apical end of the 

 primordium appears at the juncture; its hasal portion and the new 

 fine stripes arise within the fine-stripe zone. This strongly suggests 

 that the broad-stripe zone acts as an inductor, its active principle (s) 

 passing into and acting upon the narrow-stripe zone, which behaves 

 as if it were a competent responder to the inductor. We should recall 

 here that supernumerary inductor-response systems are partitioned at 

 fission so that multiplets reproduce true to type indefinitely under 

 favorable cultural conditions (Uhlig, 1960) . 



Thus far, decisive evidence that the determinative conditions are 

 confined within the cortex has not been given. The cuts and grafts in 

 Stentor and Blepharisma can hardly be expected to be limited abso- 

 lutely to the cortex. The cortex merely contains the geographical 

 markers correlated with the observed events. Tartar (1961). however, 

 reports in a preliminary way another kind of operation on Stentor 

 which points to a decisive role of the coi'tex. He made an incision and 

 withdrew all or virtually all of the endoplasm, leaving cortex and 

 nucleus. Such an operation was promptly followed by the restoration 

 of the endoplasm, normal growth, and normal reproduction. Con- 

 versely, Tartar also stripped off the entire cortex, leaving endoplasm 

 and nucleus. These cells became spherical, presumably developed a 

 surface membrane (because the remaining cytoplasm was retained), 

 and lived for some time; but they failed to regenerate cortex or to 

 develop further, eventually dying. However, if a piece of cortex was 

 left on the endoplasm, it gradually spread around the latter, recon- 

 stituted the visible markers of its gradients, and eventually regen- 

 erated and reproduced normally. These experiments indicate that a 

 small piece of cortex possesses or creates at least an essential part of 

 the morphogenetic gradients and that the cortex does not arise in 

 the absence of preexisting cortex. Unfortunately, as Tartar realizes, the 

 experiments fall short of being completely decisive. The cortex 

 might simply be providing certain mechanical or osmotic properties 

 essential for normal cellular functioning, including the production of 

 cortex itself. 



The experimental analyses on Stentor (and on Blepharisma) pro- 

 vide beautiful models of the roles in morphogenesis and cell heredity 

 of gradients, presumably in the cortex, and of inductor-response 

 systems in the cortex. The question is: Are these models generally 

 applicable to ciliates or are they limited to certain genera, for ex- 

 ample, those which show an extraordinarily well-developed capacity 

 for regeneration? Uhlig (1960) warns of the possibility of limited 

 applicability and calls for direct evidence from a variety of ciliates. 



