250 EMBRYOLOGY 



continued growth in the cells it is essential that D, the enzyme, be synthesized. 

 In order that D be synthesized it must be assumed that D is formed from the 

 union of two other substances, which we shall call E and F. But to get D syn- 

 thesized from E and F another enzyme, G, is needed. Such a process of reason- 

 ing is hopeless, for it involves the assumption of an infinite series. At each 

 synthesis another enzyme is needed. 



Self-duplicating units 



It is quite clear, therefore, that reduplication of cellular constituents 

 necessitates some self-duplicating units. These self-duplicating units must 

 participate in the following type of reaction : Two substances, A and B, form 

 C, and this reaction is catalyzed or conditioned by C itself. Substance C thus 

 becomes a self-duplicating unit which increases in amount as long as A and 

 B are supplied. Chemically speaking, this is an autocatalytic reaction, in which 

 one of the products of the reaction actually catalyzes the reaction. Some sort 

 of autocatalysis is a necessary concept in the growth of an organism. It can- 

 not be avoided. Any attempt to do so leads to the necessity of assuming an 

 infinite series of syntheses. 



When we begin to look for the smallest self-duplicating unit, we find, 

 first of all, the bacterium. But a bacterium is, like a cell, a complex mixture of 

 enzymes, and so we must search for smaller self-duplicating units. The 

 nearest approach to a self-duplicating unit at a molecular level is the virus. 

 The virus cannot reproduce itself apart from living cells. The virus cannot 

 provide its own energy source for self-duplication. We cannot culture a virus 

 in vitro. But in a living cell, with the oxidations of the living cell providing 

 the energy for synthesis, self-duplication of the virus occurs. 



What is a virus ? In general, viruses are nucleoproteins. And thus we have 

 the situation that a nucleoprotein placed in a cell will duplicate itself at the 

 expense of the energy of the cell. Normal cells have no virus infections, but 

 they have an abundance of normal-cell nucleoproteins. And if a virus can 

 self-duplicate itself in the protoplasm, it is only reasonable to assume that the 

 normal nucleoproteins of the cell can also do so. Theoretically there is no 

 reason why we may not assume that other proteins act like nucleoproteins and 

 reduplicate themselves in the cell. 



In some such manner we may also consider the reduplication of the spe- 

 cial chemical compounds necessary for differentiation. We may consider them 

 as large molecules able to build themselves up at the expense of cell oxida- 



