Growth and Differentiation in the Nervous System 285 



of the tumor on M-D cells gradually decreases in spite of the steady 

 increase in size of the transplant, which at 18 days normally fills the 

 ahdominal cavity of the embryo ( 36, 37 ) . 



A series of investigations on the effects of mouse sarcomas 180 and 

 37 on sensory ganglia of the chick embryo, explanted in vitro with 

 the hanging drop technique, showed that the neoplastic cells elicit a 

 remarkable growth effect from the sensory cells ( Figs. 32, 33 ) . The 

 effect consists of the outgrowth of a dense halo of nerve fibers from 

 the ganglia adjacent to but not in contact with the tumor within the 

 first 10 hours of culture in vitro (38). The effect is maximal if the 

 ganglia are explanted from 7- to 9-day chick embryos. It decreases 

 in ganglia explanted from older specimens and is no longer apparent 

 in ganglia from 18-day chick embryo. The effects in vitro show there- 

 fore a parallelism with the effects elicited by the tumor in the embryo: 

 In both instances the sensory cells appear to be receptive to the 

 growth agent present in the neoplastic tissue only during a limited 

 period of their growth. 



The results of the in vitro experiments showed that the tumor 

 evokes a response from the sensory ganglia even if no contact is estab- 

 lished between the two tissues. Experiments to be reported below show 

 that a growth response can be elicited from the sensory ganglia in 

 vivo by simply injecting diffusible agents into the yolk of developing 

 embryos. 



It was concluded from the above findings that a direct contact be- 

 tween nerve fibers and the tissue producing the growth agent is not 

 necessary to elicit the increase in size and number of the M-D popu- 

 lation in the sensory ganglia. 



b. Response of sensory qangtia of chick 

 embryos to diffusible 

 nerre growth factors 



It was the substitution of the tissue culture technique for the labori- 

 ous and time-consuming technique of transplantations in the embryo 

 which made possible an extensive search for nerve growth factors and 

 also offered the possibility of investigating the biochemical aspect of 

 the problem. Two potent nerve growth factors (NGF) discovered in 

 snake venom and mouse submaxillary salivary glands (39, 40, 41, 42) 

 show striking biochemical similarities with the factor isolated from 

 the tumor. In all three instances the agent isolated and purified by 

 Cohen was identified with a protein or a protein-bound particle (41). 

 Here we shall consider the biological effects of the purified extract of 



