Growtli and Differentiation in the Nervous System 289 



to a growth factor present in mouse sarcomas, snake venom, and 

 mouse submaxillary salivary glands. 



Some of the results are of particular interest for the light they shed 

 on the growth and differcntiative processes of embryonic nerve cells. 

 They show that the growth potentialities of these cells far exceed their 

 growth range under normal conditions. Furthermore, they show that 

 the active factor can reach the cells through the nerve fibers or 

 through the circulatory system. The penetration of sympathetic nerve 

 fibers into viscera and blood vessels normally impermeable to nerves, 

 as well as the hyperneurotization of exteroceptive fields with sensory 

 nerve fibers, shows that both the qualitative as well as the quantitative 

 aspects of peripheral nerve distribution are not rigidly fixed in the 

 embryo but that considerable deflections from the normal pattern are 

 tolerated. 



il. Response of sympathetic nerve cells 

 of newborn and adult mice 

 to the purified salivary NGF 



After the experiments in tissue culture indicated that sympathetic 

 ganglia of mammals (rodents and human fetuses) are receptive to the 

 NGF, we tested its effects in newborn and then in adult mice. Daily 

 injections of this agent in the amount of 2,000 biological units per 

 gram of body weight in mice between the day of birth and the nine- 

 teenth day result in a six-time increase of the volume of these ganglia 

 (42 I. The increase is due to a twofold increase in the cell population 

 and a threefold increase in the size of nerve cells (Figs. 26, 27, 30, 31) . 

 A parallel increase in the supply of sympathetic nerve fibers to the 

 blood vessels, viscera, and hair was observed. In the adult, the NGF 

 calls forth increase in the size of individual neurons but no increase 

 in their number. 



Distribution of the NGF in the organism 



Every time during the past years that we detected a new source of 

 the NGF, we hoped to have at last identified its source in the organ- 

 ism. In no instance, however, was the evidence strong enough to im- 

 plicate the tissue as the "source" of the NGF, and the search is still 

 in progress at the present time. One interesting and perhaps revealing 

 observation was to find that the NGF is present everywhere in the 

 organism (44). It is harbored in exceptionally high amount in the 



