BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 483 



gastrulse: it would appear that the presence of an abnormal nucleus in an 

 egg makes it impossible for the embryo to produce substances which it 

 needs for development, while, in the case of the heated eggs, the high tem- 

 perature would inactivate morphogenetic substances. But, in both cases, 

 these substances diffuse easily from a normal host into the graft. 



In respect of PNA metabolism also, the lethal hybrids behave very much 

 like the heated gastrulse: there is almost no PNA synthesis in the blocked 

 embryos, while the controls steadily increase their PNA content (Steinert^^) . 

 Also, when a graft of a lethal hybrid is being revitalized under the influence 

 of a normal host, its cells undergo a considerable increase in basophilia. 



It may thus be said, in conclusion, that PNA synthesis and embryonic 

 development are so closely linked together that no way has been found so 

 far to dissociate the two processes ; there is no doubt either that the integrity 

 of the normally existing PNA gradients is essential for normal morpho- 

 genesis. 



Let us now consider whether PNA or pentosenucleoproteins, including 

 among them the microsomes, play a special role in the all-important mor- 

 phogenetic process, neural induction. 



3. Pentose nucleoproteins and Neural Induction 



It is a well-known fact that the nervous system arises, in normal de- 

 velopment, under the inducing activity of the underlying organizer 

 (chordomesoblast) : presumptive epidermis will differentiate into nervous 

 system if it is placed in contact with the organizer, even when the latter 

 has been killed by heat or alcohol treatment. However, the inductions ob- 

 tained with killed organizers — or with many tissues coming from a wide 

 variety of animal species — are often less harmonious than the normal ones : 

 for this reason, abnormal inducing agents are often called evocators and the 

 reaction which they produce is called an evocation, to distinguish it from a 

 normal induction (for a detailed discussion of these questions, see 

 Needham^i and Brachet^^). 



The fact that evocators are so widespread in the animal kingdom has 

 led to many attempts to isolate an active "evocating substance." The re- 

 sults have been extremely disappointing, because it was soon apparent 

 that many pure and chemically unrelated substances are equally active. 

 The presumptive epidermis becomes neuralized as an unspecific reaction to 

 many stimuli, very much as the unfertilized egg responds in an unspecific 

 manner to parthenogenetic stimuli. As pointed out by Waddington, Need- 

 ham, and Brachet,*- it is very likely that the normal inducing substance 



*' J. Needham, "Biochemistry and Morphogenesis." Cambridge Univ. Press, New 



York, 1942. 

 *''■ C. H. Waddington, J. Needham, and J. Brachet, Proc. Roy. Soc. (London) B120, 



173 (1936). 



