212 The Nature of Biological Diversity 



of the determinative parts of the pattern of the mosaic egg of Xenopus. 

 In such eggs, the different parts of the egg cortex are segregated into 

 different cells during cleavage, presumahly in the absence of growth 

 or further cortical developments. When growth and further develop- 

 ment begin, the cells endowed with diverse cortices appear to follow 

 different lines of cortical development (cf. Sonnehorn, 1960). Juxta- 

 position of cells with different cortical patterns leads to organizer and 

 inductor-response reactions. The parallels to regionally diverse cor- 

 tical parts in ciliates, and the interactions between those that are 

 adjacent, are too obvious to be labored. The main difference between 

 unicellular and multicellular organisms in these respects is in the 

 nature of the differentiated and interacting units: different regions 

 of the cortex of one Protozoan cell correspond in principle to the 

 cortices of different cells in the Metazoa (Faure-Fremiet, 1954). 



Finally, in Metazoa as in Protozoa, the cortical patterns and events 

 undergo a progressive self-directed secpaence of changes, during the 

 clonal life cycle in the latter and during the individual life cycle in 

 the former. In both, one change leads to the next and the sequence 

 becomes a cycle by a return to the starting point. In the ciliates, any 

 cell (after immaturity and before advanced senescence) can undergo 

 fertilization (or encystment or physiological regeneration) and re- 

 turn to the cortical starting point. Likewise all cells capable of division 

 may be able to do this in certain plants (Steward, 1961) . Only certain 

 cells can do it in the Metazoa. In the Protozoa and the Metazoa, the 

 return may be rapid and normally confined to a cell that has under- 

 gone fertilization or one specialized to undergo it. The many parallels 

 suggest that the roles of the cell cortex and the principles of their 

 operation may be fundamentally alike in unicellular and multicellular 

 animals. 



This basic similarity implies an evolutionary development of cor- 

 tical specificities; and the considerable degree of autonomy of cortical 

 parts suggests a corresponding degree of independence in their evolu- 

 tion. This is strongly indicated by the existence of a number of genera 

 of flagellates and ciliates (for example, Giardia, Teutophrys) in which 

 the animals appear morphologically to be doublets or higher multiples 

 of the animals of other genera (Faure-Fremiet, 1945). Within this 

 limited group, the evolution of genera clearly runs parallel to the 

 laboratory production of hereditary doublets and multiplets. 



In general, there is no doubt about the existence of cortical evolu- 

 tion in the ciliates. The whole taxonomy of this group hinges upon 

 cortical characteristics (Corliss, 1961). However, nearly all of the 

 gross morphological features of ciliates are cortical features. The 



