506 



CHAPTER 41 



oral segment gave rise to a strain having a 

 complete extra oral segment including an 

 extra vestibule, mouth, and gullet. Other 

 studies reveal that various experimental 

 modifications of visible cortical organization 

 are perpepuated during cell reproduction, 

 and that certain visible cortical structures 

 initially absent, do not arise de novo. 



These and other results establish the im- 

 portance of the cortex in differentiation. 

 The cortex is not completely autonomous, 

 however, since some nuclear genes are 

 known to determine visible cortical struc- 

 tures or their morphogenesis. As already 

 mentioned, a small additional piece of cor- 

 tex can give rise to cortical changes of 

 greater degree. Clearly then, the nature 

 and action of the cortex is dependent not 

 only upon its own composition but upon 

 nuclear genes and their products as well as 

 metabolism in general. It should be noted 



that double-stranded DNA has been re- 

 ported in human erythrocyte membranes. 

 This DNA has a molecular weight of about 

 I0 ,; and a G + C content of approximately 

 39-429f. Its homogeneity (and its possi- 



A 4- T 

 ble higher r ratio) suggest that this 

 C + G 



is not merely adsorbed DNA. At present, 

 the mode of operation of the cortex can 

 only be described in general, largely specu- 

 lative, terms. "The much more difficult 

 task for the future is to define and specify 

 in molecular terms the decisive structures, 

 gradients, and inductor-response systems 

 and to reveal how specific absorption, ori- 

 entation, and activation of migratory mole- 

 cules leads to visible morphogenesis and ge- 

 netic stability of cell organization." (T. M. 

 Sonneborn, 1963). 



- 1 See L. Philipson and O. Zetterqvist (1964). 



SUMMARY AND CONCLUSIONS 



The synthesis of mature ^>T4 progeny is regulated by blocks of parental viral genes 

 which function at the same or nearly the same time and which are arranged in the 

 circular linkage map in a sequence reflecting successive stages of phage morphogenesis. 



Nuclear transplantation, somatic cell mating, antibody-antigen, and biochemical em- 

 bryological investigations are revealing the genetic and molecular bases of differentia- 

 tion and development. 



Studies of thymidine kinase indicate that mitosis involves cyclical gene action. Base- 

 specific RNA plays an intracellular role in learning and an intercellular role in differ- 

 entiation. Steroids seem to be involved in RNA production and histone distribution. 



Although the chemical mechanism is unknown, studies of Paramecium reveal the 

 importance of the cortex in differentiation, and emphasize that morphogenesis depends 

 upon both the nuclear genetic material and the remainder of the protoplasmic and 

 metabolic environment. 



REFERENCES 



Cytogenetics and Developmental Genetics, Amer. Zool., 3 (No. 1), 1963. 



Demerec, M., "Clustering of Functionally Related Genes in Salmonella typhimurium" 

 Proc. Nat. Acad. Sci., U.S., 51:1057-1060, 1964. 



Differentiation and Development, Boston: Little, Brown, 1964; and J. Exp. Zool., 157, 

 No. 1, 1964. 



Doi, R. H., and Igarashi, R. T., "Genetic Transcription during Morphogenesis," Proc. 

 Nat. Acad. Sci., U.S., 52:755-762, 1964. 



