PHYSICAL PROPERTIES OF THE INDUCING AGENT 81 



These striking experiments of Raven (1938) have led Dalcq (1941) 

 and Needham ( 1 942) to the very interesting hypothesis that the action 

 of the inducing agent might be similar to that of viruses. The well- 

 known fact that the medullary plate which has been induced by the 

 organizer acts as an inductor if it is grafted into the blastocele of a 

 young gastrula leads to the same conclusion. It looks as if the in- 

 ducing agent can, hke a virus, "infect" the neighboring cells, prop- 

 agate and migrate from one cell to another. A further suggestion 

 has been made by the author (1949): the hypothetical virus might 

 be identical with the microsomes, which have dimensions and a 

 RNA-content comparable to those of many viruses and might there- 

 fore possess genetic continuity. 



A number of experiments have been performed in recent years 

 in order to test these hypotheses. As we shall now see, they have so 

 far failed to give clearcut answers. 



The most direct experiment carried out to test the "microsomes- 

 virus hypothesis" was the isolation of microsomes by ultracentrif- 

 ugation of homogenates and the microinjection of these particles 

 into a ventral blastomere of a young morula. Such experiments have 

 been attempted by Brachet and Shaver (1949) and by Brachet et al. 

 (1952). But the results were rather disappointing, although a local 

 increase of basophilia in the injected blastomeres was often ob- 

 served. Very few embryos, out of several hundreds, formed a nerv- 

 ous system on the ventral side and it is likely that these few cases 

 resulted from purely mechanical troubles of the gastrulation move- 

 ments rather than from true induction. It should, however, be added 

 that the experimental conditions adopted for the isolation of the 

 microsome pellet were far from ideal ; the temperature in the ultra- 

 centrifuge was relatively high and saccharose was not added to the 

 homogenization medium. 



Very recently, Yamada (personal communication) has isolated 

 microsomes under much better isolation conditions from amphib- 

 ian embryos. Adding them to small ectodermic explants, he found 

 that they have a strong inductive (archencephalic) activity. 



Some evidence for the view that substances of high molecular 

 weight can diffuse (if only for a short distance) from cell to cell in 



References p. 90193 



