214 The Nature of Biological Diversity 



aside from trichocysts. Flagellates have a cortex even when the kineto- 

 somes lie in the endoplasm; it may he little more than a limiting 

 membrane system. Perhaps the outermost membranes of Paramecium 

 are homologous with the outer membranes of flagellates. The well- 

 known inducible shedding of the pigment-containing "pellicle" in 

 Blepharisma (Nadler, 1929) and Stentor (Tartar, 1961) without loss 

 of essential nonreplaceable cortical components, taken in connection 

 with Tartar's earlier cited result on the irreplaceability of the Stentor 

 cortex, seems to weigh against that part of the Ehret and Powers 

 (1959) conception which reduces the entire cortex to elements having 

 internal origin. 



The account just given indicates that new parts were added to the 

 cortex during the evolution of the Protozoa. Knowledge is lacking as 

 to whether certain cortical parts or the whole cortex is the seat of the 

 gradients and physiological diversities that direct the course of mor- 

 phogenesis and the processes which result in the inheritance of cortical 

 characteristics. In the Metazoa, as we have seen, gross morphological 

 differentiation of the cortex has largely disappeared; but the under- 

 lying determinative physiological differentiations of the cortex are 

 present in highly developed form, as Curtis's (1960) experiments 

 demonstrate. This leads to the guess that the basic genetically and 

 morphogenetically important part of the cortex is not the part that 

 is unique to the Protozoa, the part bearing the morphologically visible 

 differentiations. 



C. The role of preformed cortical structure 



The question with which this chapter began — whether the structure 

 of a cell (aside from that of its chromosomes) plays an essential or 

 nonessential role in the determination of the structure of its cell 

 progeny — has been answered unambiguously by experimental analysis. 

 Such structure may play little or no part in many essential intracel- 

 lular activities which depend only on the physicochemical properties, 

 amounts and random collisions of newly imbibed food (in the broad- 

 est sense), and newly produced direct and indirect products of gene 

 action. On the other hand, our study of Paramecium — in general 

 agreement with a number of other studies — shows that preformed 

 cortical differentiations are essential for their own reproduction. 

 Certain visible cortical structures failed to arise de novo when they 

 were initially lacking. Experimental modifications of the visible cor- 

 tical organization were perpetuated during cell reproduction. Thor- 

 ough breeding analysis, combined with other accessory modes of 



