V CONCLUSIONS CONVERGENCE OF CONCEPTS 537 



Nickerson and Falcone (19563, b) showed that the normal yeast possesses an 

 enzyme system capable of reducing disulfide bonds in a cell-wall mannanprotein 

 complex whereas the divisionless mutant is deficient in this activity. Budding and 

 subsequent proliferation occurred only if the cell-wall protein was depolymerized 

 subsequent to reduction of its SS groups. The noteworthy series of investigations on 

 the metabolic control of morphogenesis in Blastocladiella carried out by Cantino 

 (1955) and his associates as well as the exploration of the mechanism of spore 

 formation in bacteria (reviewed by Foster, 1956) may point to similar relations of 

 protein aggregation and the metabolic activity of the cell. 



From the point of view of animal morphogenesis these findings are potentially 

 significant in two respects. They may be directly pertinent to an understanding of 

 protein polymerization in metabolically controlled morphogenetic processes such 

 as formation of ciliae in the apical tuft of sea urchin larvae, which is regarded as 

 index of "animalization" and known to depend on the metabolic state of the ani- 

 malized cell. It cannot be excluded as yet, however, that metabolic processes, simi- 

 lar to those which aflfect the state of proteins involved in these polymerizations, 

 also aflfect the enzymatic units of the PFS, such as enzymes similar to the pro- 

 teases, or influence the structure of "templates" and in this way modify indirectly 

 the structure of the processed proteins. Ranzi and his associates (Ranzi, 1953; 

 Ranzi, 1955; Ranzi and Citterio, 1955) have initiated a series of studies aimed at 

 defining the state of aggregation of embryonic proteins and their alteration after 

 exposure of the embryos to agents, such as lithium, which are known to influence 

 markedly the course of development. 



Obviously, the interaction of proteins with each other, with nucleic acids, 

 mucopolysaccharides, or lipoids is essential in the formation of larger structural 

 units such as the endoplasmic reticulum, mitochondria, and the cell surface. The 

 role of the first two entities in embryonic differentiation has been repeatedly 

 touched upon in this chapter. The significance of the cell surface as a large mole- 

 cular aggregate should be given added emphasis in this context. First it should be 

 pointed out that the identification of protein as a structural unit in the cell 

 surface has recently been followed by a remarkable variety of methodological 

 approaches. Such are the use of proteolytic enzymes for the separation of tissues 

 (Herrmann and Hickman, 1948a) and of cells (Moscona, 1952 and Moscona and 

 Moscona, 1952) implicating proteins as the main component in the system 

 maintaining the coherence of cells, localization of enzymes in the cell surface 

 (Rothstein and Maier, 1949), the analysis of surface groups of bacterial cells 

 responsible for the attachment of virus particles (Tolmach, 1957), and the im- 

 munological analysis of bacterial cell surfaces (Vennes and Gerhardt, 1956). 

 Apart from this purely structural consideration of the cell surface, Christensen 

 (1955a, b) suggests that the cell surface is not only the site of amino acid activation 

 connected with their transfer into the interior of the cell but represents possibly a 

 site of protein synthesis. Such a possibility may seem plausible in view of electron 

 microscopic observations revealing a continuity of the endoplasmic reticulum, 

 with its microsomal derivatives, and the cell cortex. Since microsomes are 

 apparently the main site of protein synthesis the inclusion of microsomal material 

 in the cell cortex could indicate a similar function. One is also tempted to speculate 



Literature p. 3jg 



