Differentiation of Neuroblasts 57 
quite intact. ‘This fact, taken in connection with the constant 
and apparently immediate regeneration, makes it necessary to 
remove the limb a number of times before there is any considerable 
difference in the amount of musculature on the two sides. But 
the differentiation of the nervous system is equally slow and it is 
possible to materially decrease the muscular mass before the motor 
nerves are well developed. The toad was treated in the same way, 
but regeneration occurs more slowly, and differentiation of the 
nervous system more rapidly, so that I have no specimen in which 
the limb was removed more than twice. With the frog, I shall 
mention only experiments in the removal of somites. 
The methods used to ascertain the extent of the defect in the 
nervous system were the same as those employed in studying the 
chick embryo. 
C Experiments on Amblystoma 
Experiment 25. The left hind-leg bud was first removed May 
3 and again successively on May 22 and June. The larva was 
preserved June 5. Some idea of the difference in the two limbs at 
this time may be obtained from Fig. 25. ‘The bones of the pelvis 
are present but smaller on the operated side, and a small portion of 
the femur is distinguishable, but there is no evidence of any of the 
other bones of the limb. For the purpose of the present problem 
it will be convenient to divide the muscles innervated by the 
sciatic plexus into three groups: (1) The proximal muscles of the 
thigh; (2) the muscles of the leg proper; (3) the muscles of the 
bladder and its immediate neighborhood. In this experiment, 
groups (1) and (3) are uninjured. Of group (2), there is a consid- 
erable muscular mass, but it is impossible to identify individual 
muscles, so that it can only be said that the amount of muscula- 
ture is much decreased. ‘The sensory area is, of course, also 
decreased. 
The sciatic plexus is formed from the union of three nerves as 
shown in Fig. 43, 4, drawn from the normal side of this larva. 
Nerve I innervates mainly the proximal portion of th. thigh, and 
as this region is uninjured, only enough of its course 1s given to in- 
dicate the branch which joins the sciatic plexus. B drawn on the 
