THE ROLE OF RIBOXUCLEIC ACID AND SULFHYDRIL GROUPS 259 



The possible role of microsomes in induction 



We owe to Raven ( 1938 ) a demonstration of the important fact 

 that the inducing principle can diflFuse from cell to cell: if a non-induc- 

 ing fragment of presumptive ectoderm is left for some hours in contact 

 with the living organizer, it acquires inducing capacities. These strik- 

 ing experiments of Rav^en led Dalcq ( 1941 ) and Needham ( 1942 ) 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 

 inducer if it is grafted into the blastocele of a young gastrula leads to 

 the same conclusion: it looks as if the inducing agent, like a virus, can 

 "infect" the neighboring cells, propagate, and migrate from one cell to 

 another. A further suggestion has been made by the author ( 1949 ) : the 

 hypothetical "virus" may be identical with the microsomes, which have 

 dimensions and an RNA content comparable to those of many viruses 

 and may therefore possess genetical continuity. 



A number of experiments have been performed in recent years to 

 test these hypotheses: as we shall now see, they have so far failed to 

 give clear-cut answers. 



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

 virus hypothesis" was the isolation of microsomes by ultracentrifugation 

 of homogenates and the microinjection of these particles into a ventral 

 blastomere of a young morula. Such experiments have been attempted 

 ( Brachet and Shaver, 1949; Brachet et al., 1952 ) , but the results were 

 rather disappointing, despite the fact that a local increase of basophilia 

 in the injected blastomeres was often observed. \^ery few embryos, out 

 of several hundreds, formed a nervous system on the ventral side, and 

 it is likely that this resulted from purely mechanical troubles of the 

 gastrulation movements rather than from true induction. It should, how- 

 ever, be added that the experimental conditions adopted for the isola- 

 tion of the microsome pellet were far from ideal: the temperature in 

 the ultracentrifuge was relatively high, and saccharose was not added 

 to the homogeneization medium. 



Very recently Yamada (personal communication) has isolated 

 microsomes under much better isolation conditions from amphibian 

 embryos. Adding them to small ectodermic explants, he found that they 

 have a strong inductive ( archencephalic ) activity. 



Finally, it has been reported, also very recently, by Ebert ( 1959 ) 

 that addition of microsomes isolated from muscles can induce the dif- 

 ferentiation of nerve fibers in chorio-allantoic membrances of chick 

 embryos, provided that a virus which can infect the chorioallantoic 

 membrane cells is added together with the microsomes. The role of the 

 virus would simply be to facilitate the penetration of the microsomes 



