208 The Nature of Biological Diversity 



experiment that might yield the necessary information is induction 

 of "picked" cortex (page 181). Cytological study should reveal 

 whether the effective picked pieces contain an endoral kinety, a ves- 

 tibule-gullet contrast juncture, only one part of that system, or 

 merely some pattern of cortical structure which is decisively different 

 from that of the area into which it is implanted. 



While awaiting decisive evidence as to the possible roles of the 

 endoral kinety and vestibule-gullet juncture, the available facts con- 

 cerning the general situation point to a tentative choice. Kineties 

 genetically differentiated for the production of oral apparatuses (or 

 for any other visible structure) do not exist in the decisively analyzed 

 ciliates. Instead, interacting gradients, visibly expressed as juxtaposed 

 different cortical areas, have proved to be determinative. In Para- 

 mecium, the determination of at least one structure — the cytopyge — 

 occurs in the same way. In the absence of contrary decisive evidence 

 in any ciliate and in the light of supporting evidence for the principle 

 of interacting gradients in all well-analyzed cases ( including one in 

 Paramecium), tentative rejection of the interpretation of genetically 

 differentiated kineties, including the endoral kinety of Paramecium, 

 seems justified. For the same reasons, tentative adoption of the prin- 

 ciple of determination of morphogenesis by interacting gradients or 

 fields seems justified, even when they are not correlated with visible 

 pattern junctures, as in the ciliate Glaucoma (Frankel, 1960a, b, 

 1961). 



Up to this point, the problem under discussion has been the part 

 played by the cortex in the perpetuation through fissions of its own 

 specific pattern of differentiation. However, the discussion has pro- 

 ceeded upon a tacit assumption which now needs to be made explicit 

 and to be subjected to careful scrutiny. Tacitly it has been assumed 

 that the processes of morphogenesis at fission partake of an all-or-none 

 character. Either a structure develops or it doesn't. And this is deter- 

 mined by the interactions of a few primary gradients at junctures of 

 gradient contrast. So relatively simple a model is a good first approxi- 

 mation, but the impression of its adequacy comes from putting aside a 

 large class of facts which will now have to be examined. 



To begin with, the reader will find on page 193 an account of a 

 series of cortical deviations from a complete doublet type which led 

 to eventual complete and irreversible loss of vestibule, mouth, and 

 gullet on one of the two interpolar oral segments of doublets, leaving 

 all other landmarks of this segment. A number of stages in the loss 

 were observed. The process thus occurred gradually over a series of 

 successive fissions. Observations of comparable progressive cortical 



