486 J. BRACKET 



the eye-cup have since been reported by McKeehan." They prove conclu- 

 sively that induction cannot be mediated by simple, easily diffusible, chem- 

 ical substances; close contact between the organizer and the induced tissue 

 is a prerequisite to successful induction. It has also been found (Brachet^^) 

 that neutral red, when used as a vital dye for amphibian gastrulae, stains 

 only the cytoplasmic granules; when, however, a stained explant is inti- 

 mately joined to an unstained one, the dye diffuses in the latter. Such a 

 diffusion is effectively hindered by the interposition of a cellophane mem- 

 brane between the explants, although the membrane is permeable to neutral 

 red when in solution. 



These findings certainly lend support to the view that normal induction 

 is mediated by large molecular aggregates able to cross cell membranes; 

 they do not prove that the microsomes are involved in the process, but the 

 cytochemical evidence reported earlier in this chapter favors this view. 



Summing up, it may be said that there is no doubt that PNA, probably 

 in the form of microsomes, plays an important part in morphogenesis; but 

 the elucidation of the mechanism of its action, especially during neural in- 

 duction, remains a task for the future. 



III. PNA and Protein Synthesis 



The conclusion that PNA is somehow concerned with protein synthesis 

 was reached independently by Caspersson^^ and the writer-® some twelve 

 years ago; it was mainly based on cytochemical results, which have been 

 largely substantiated by direct chemical analysis. After a brief analysis of 

 the cytochemical and biochemical evidence on which the hypothesis rests, 

 we will discuss present ideas about the mechanisms of protein synthesis, 

 first at the cellular, then at the biochemical, level. 



1. Cytochemical and Biochemical Evidence for a Link 



BETWEEN PNA AND PrOTEIN SYNTHESIS 



o. Cytochemical Evidence 



The considerable amount of evidence obtained by Caspersson and his 

 co-workers in favor of the idea that all protein synthesis needs the presence 

 of nucleic acids has recently been summarized in book form by Caspers- 

 son;^^ the main results obtained by the writer can be found in his original 

 paper-® and in two more recent books. '^'^^ 



Caspersson's results were obtained with his very elegant method of 

 ultraviolet microspectophotometry, while most of the work coming from 

 other laboratories is based on studies made with basic dyes (usually Unna's 



" M. S. McKeehan, J. Exptl. Zool. 117, 31 (1951). 



'* T. Caspersson, Naturwissenschaften 28, 33 (1941). 



65 T. Caspersson, "Cell Growth and Cell Function." Norton, New York, 1950. 



