Role of Preformed Structure in Cell Heredity 211 



B. Multicellular organisms and 

 cortical evolution 



Double animals, parabiotic twins, and tbc like are — as everyone 

 knows — commonplaces of the experimental embryology of Metazoa. 

 But they are not reproduced. This is because the individual cells are 

 not changed and reproduction is from a single cell or a fusion of two 

 cells. Only the pattern of cellular association is altered. However, the 

 cellular association itself functions genetically in the asexual repro- 

 duction of certain Metazoa and these can form hereditary doublets 

 as stable as those in ciliates. Sonneborn ( 1930 ) obtained two differ- 

 ently oriented doublets of the Rhabdocoel turbellarian, Stenostomum 

 incaudatum. Each reproduced true to type indefinitely through asex- 

 ual reproduction. These cases are particularly instructive not only 

 because they are in Metazoa, but especially because detailed com- 

 parison with the doublets in ciliates reveals so much about the basic 

 similarities. In both the ciliates and Metazoa, the gross reproduction 

 true to type involved indirect, dynamic processes quite independent 

 of any template processes of replication per se. In both, the basis lay 

 in the perpetuation of a new combination of the number and the 

 arrangement of parts which were indirectly self-reproducing. In both, 

 the perpetuation depended upon inductor-response systems of mor- 

 phogenesis with complex development of the hereditary structural 

 parts in each generation. The chief difference lay in the units of 

 organization: cells and tissues in the case of the Metazoa. parts of the 

 cortex of a single cell in the case of the ciliates. 



What do the Metazoa have to tell us about the role of the cell 

 cortex? The literature of experimental embryology of higher animals 

 is replete with evidences of the importance of the cell cortex. It plays 

 decisive roles, for example, in cell "recognition" during reaggregation 

 of dissociated cells (Moscona, 1957 I , in organizer and inductor actions 

 and the response to them by competent cells (Weiss, 1939), in the 

 mosaic of determinative regions of the egg ( Dalcq and Pasteels, 1937, 

 1938; Curtis, 1960), and presumably in the whole distinctive molecu- 

 lar organization of each cell type (Weiss, 1962, and his earlier papers 

 on molecular ecology and related matters ) . The conclusions set forth 

 above as to the existence and roles of gradients, fields, interactions 

 between parts, and movement or differential growth in ciliate morpho- 

 genesis and cell heredity have their counterparts in the older and 

 much more extensive experimental embryological studies on Metazoa. 



Recently Curtis (1960) has demonstrated the cortical localization 



