Dijjerentiation of Neuroblasts 61 
E Removal of Somites in Rana Pipiens 
I thought it possible that the capacity of the frog for regenera- 
tion might not extend to the reproduction of a whole somite, and 
I therefore tried removing these as a means of getting at the initial 
stages in the development of the nerve fibers. For this purpose 
larvae were removed from the egg capsules from 24 to 36 hours 
before hatching, and from 1 to 3 somites completely removed. It 
is perfectly easy to do this without injuring other parts, but the 
animals all died three or four days later; however, by leaving a 
small amount of tissue close to the spinal cold and notochord they 
develop as rapidly as the normal larve, and after a very few days 
there is no noticeable external defect. 
On sectioning these the loss is evident, but regeneration and 
probably re-differentiation of adjacent parts is found to occur, 
for no muscles are missing, but all are decreased in size and this 
loss extends to somites above and below the injured region. This 
process is well advanced before there is any well-defined differen- 
tiation of the medullary tube, so that again it is impossible to 
ascertain the effect of the complete lack of the end organs of any 
nerve. Fig. 31 shows the condition of the musculature in a speci- 
men preserved two months after the removal of the somites, and 
Fig. 32 shows the spinal cord of this section more highly magnified. 
The defect in the ventral portion of the spinal cord is evident 
throughout the affected region, and comparison with older larve 
shows that neither the motor horn nor the ventral roots have 
attained complete development. ‘These were also examined for 
signs of degeneration, but none were found, so that these results 
again confirmed the conclusions reached by experiments on Ambly- 
stoma. 
F Summary of Conditions Observed in Amphibians 
1 Ifa limb bud of Amblystoma tigrinum or Bufo americanus 
is removed the following conditions are found: 
a_ If the regeneration is rapid and extensive the development 
of the nervous system is apparently normal. . 
b If the regeneration is slight, or if the limb is repeatedly 
