Warren Harmon Lewis 141 
the chorda and muscle develop into much more normal-appearing tissue 
than the rudiment of the nervous system. ‘The latter, though prede- 
termined at this time, does not seem to be able to differentiate into per- 
fectly normal tissue after its transplantation. It is thus probably de- 
pendent on certain, as yet unknown, relations in its normal environment 
for its more perfect differentiation. Just what these relational factors 
are or for how long a period they must act is not clear, but a some- 
what later stage after the first faint indications of the medullary plate 
appear the nervous tissue or its rudiment possesses great power of 
self-differentiation. 
