204 The Nature of Biological Diversity 



performed, mainly on Stentor (Tartar, 1961; Uhlig, 1960) and on 

 Blepharisma (Suzuki, 1957) — show clearly that no kinety or kinety 

 field is alone endowed with the capacity to be the site of formation of 

 the complex peristomial primordium. This can happen anywhere 

 around the whole circumference of the body when a juncture of con- 

 trast in stripe (and kinety) widths is set up by cuts and/or grafts. 

 Apparently any kinety or group of kineties can serve as the site of 

 primordium formation. The kineties can be no more than instruments 

 of morphogenesis; they are not its cause. 



What then is the cause of specific morphogenesis? Tartar's results 

 seem to prove that the origin and development of the peristomial 

 primordium is determined by interaction between adjacent narrow- 

 and wide-striped cortical areas. As he and Uhlig (1960) note, the 

 visible gradation of stripe widths around the body is the visible ex- 

 pression of a gradient with its extremes meeting on the ventral surface 

 where the apical end of the primordium normally arises. They showed, 

 however, that not all artificially produced junctures of different stripe 

 widths serve to induce primordium formation. If two junctures of 

 unequally diverse stripe contrasts are created, the lesser contrast may 

 fail to induce a primordium, especially if it is close to the greater 

 contrast. Even if only one juncture exists and the stripe contrast is 

 extremely slight, it may fail to induce. There are thus degrees of 

 interaction at junctures correlated with difference in the gradient 

 levels, and degrees of inhibition of primordium formation diffusing 

 in gradient fashion from a major juncture. Another index of gradient 

 action is the delineation during fission of new fine stripes in the fine- 

 stripe ramifying zone: the number of new stripes decreases with dis- 

 tance from the broad-stripe region. 



The Stentor work shows that the anteroposterior position of the 

 primordium and the differentiation of its parts are determined by 

 another gradient extending from the foot or base toward the apex. 

 This gradient, for example, determines where the primordium bends 

 to the right across the fine-stripe ramifying zone. Further, the differ- 

 entiation of a mouth at the base of the primordium depends upon 

 the presence of a foot. Supernumerary feet in abnormal positions 

 induce the formation of supernumerary mouths in abnormal posi- 

 tions; absence of a foot results in failure to form a mouth. Altogether, 

 then, the origin and development of the peristomial primordium 

 depend upon the interactions between the two gradients. 



Two facts already mentioned give a most important clue to the 

 general nature of operation of the first gradient. Although an adjacent 

 wide stripe zone is essential, all the new formations appear either at 



