476 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



but only for his initial activation. Such a release mechanism probably implies 

 enzymatic activities, of which we perceive some signs by the detection of alkaline 

 phosphatases. In the idea that some ribonucleoproteins could be reponsible for 

 induction, Flickinger (1956) has attempted to explain their origin. This author 

 draws the attention on the relation of phosphoprotein-phosphatase activity to 

 yolk platelet utilization in amphibian eggs. He suggests that livetin and its asso- 

 ciated RNA are released from the yolk platelets to afford the inductive agents. 

 We did see here that several arguments, as well experimental (p. 343) as 

 biochemical (p. 361) favor this idea of the participation of yolk in morphochoresis. 

 If such is the case, mammals, where yolk is not longer present, should be studied 

 in order to find the biochemical devices which they have substituted to the use 

 of yolk derivatives. The remarkable increase of RNA in the embryonic knot of 

 mammals, and the coincident appearance of alkaline phosphatase, further the 

 gradient field of RNA in the ectophyll are to be kept in mind. 



A direct study of the primary inducing agent (s), presumably without alteration 

 of these, has been attempted by Niu (1956) by mass cultures of small pieces of 

 organizer-tissue, explanted with a minimum of coat. The active macromolecules 

 are presumed to pass into the "conditioned" medium (p. 408), and its content was 

 used to culture competent ectoblast and was submitted to biochemical examination. 

 A sufficient quantity of such media could not have been gathered from hanging 

 drop cultures, but was obtained by cultivating 20-30 pieces of organizer-tissue 

 in the well of depression-slides. "Seventy such slides prepared at one time will 

 yield, after 7-10 days, 5-6 ml of the conditioned medium. . . The advantage of 

 this method is that after withdrawal fresh salt solution may be added to the culture 

 and 7-10 days later the medium may be withdrawn again. This process is repeated 

 4-6 times". In this way, conditioned media from the ist to the 6th order could 



1000 



230 



250 



Fig. 100. A chart of U. V. absorption spectra of the conditioned media collected from 



deep-well cultures of ^. ligrinum dorsal lip. ist to 6th, the conditioned media from the first 



through the sixth order (see text). From Niu, 1956. 



