Growth and Differentiation in the Nervous System 287 



the mouse salivary glands on the sensory ganglia of chick embryos. 

 Similar results were obtained by injecting the purified agent extracted 

 from snake venom. 



The addition to the culture medium of the purified protein ex- 

 tracted from the salivary glands or from the snake venom elicits the 

 same nerve growth response as fragments of tumor or tumor extract. 

 Both the snake venom and the mouse salivary glands harbor this 

 agent in such a large amount that a very small quantity of the puri- 

 fied extracts is sufficient to evoke the in vitro effects. It was shown 

 by Cohen that the purified salivary extract is active at a concentra- 

 tion of 0.01 gamma of protein per milliliter (41). This solution, when 

 added to the medium of culture in the proportion of one part to one 

 part of synthetic medium and one part of plasma, elicits a dense halo 

 of nerve fibers from the explanted sensory ganglia. It was designated 

 as one biological unit. A comparison between the potency of the tumor 

 and the salivary gland extract indicates that the latter is about 6,000 

 times more potent on a dry weight basis than the tumor. 



Daily injections of a few gamma of the purified protein isolated 

 from mouse submaxillary salivary gland into the yolk of 6- to 10-day 

 chick embryos call forth a growth response from the M-D population 

 of sensory nerve cells, comparable to the response elicited by intra- 

 embryonic transplants of mouse sarcomas (Figs. 20, 21). The size 

 increase of this population has its counterpart in the hyperneurotiza- 

 tion of the exteroceptive embryonic fields. Since no increase in the 

 V-L population was observed and no hyperneurotization of the mus- 

 cles is apparent, it is tempting to correlate these two populations with 

 different fields of distribution. Further investigations are in progress 

 to elucidate this point and to trace the distribution of nerves from 

 the hypertrophic ganglia to their terminal structures. 



injected for 1 week with the NGF salivary factor; C. control ganglion. FIG. 29. 

 Transverse sections through superior cervical ganglia of 4-month-old mice. C, con- 

 trol; E, mouse injected for 5 days after birth with the antiserum to the salivary 

 NGF. FIGS. 30 and 31. Sympathetic nerve trunks of control and experimental chain 

 ganglia of Fig. 26 at higher magnification. Low arrows in Fig. 26 indicate areas 

 enlarged in Figs. 30 and 31. FIG. 32. Spinal ganglion of a 7-day-old chick embryo 

 cultured in vitro for 24 hours. FIG. 33. Spinal ganglion as in Fig. 32, combined in 

 vitro with a fragment of mouse sarcoma 180. Note the dense fibrillar halo. FIG. 34. 

 Sympathetic ganglion of a human fetus 3 months old cultured for 24 hours in a 

 medium to which the NGF salivary factor was added. 



