22 Indications of Continued Formative Influence 
entiated yolk cells and the cells of the already completely 
differentiated germinal layer. With the elimination of 
other possibilities this leads us to the conclusion that the 
progressive differentiation is accomplished in material 
already in position before differentiation commenced, and 
remaining there throughout it, and therefore in passive 
yolk material by direct transformation of the yolk cells 
(accompanied in the case of the ectoderm and mesoderm 
also by the division of these cells).” 
“As to the location of the causes of these processes,” 
continues Roux, ‘“‘we can draw a few further conclusions.” 
“Since the yolk cell material later differentiated to form 
the germ layers in the manner described above has been 
quite disordered in its substance forming the bodies of the 
cells, by the operation, and since also the nuclear material 
of the cells which are later formed from it has never 
yet taken its place by virtue of a typical division, but, being 
derived partly from the nucleus of the half operated upon 
and partly from the emigrated naked nuclei of the half 
remaining intact, owes its disposition to the chance of 
the moment, therefore the conception, possible in the 
case of normal development that at typical places there 
is always deposited typical material capable of quite 
definite independent development, cannot be admitted in 
this case.” 
“We must conclude rather, that the cause of this typ- 
ical formation of the germ layers of the first developed 
half of the egg, extending into the half operated upon, lies 
in forces which proceed from the germ layers of the first 
half.” 
“I conclude, then, that in our postgeneration the 
progressive differentiation extends in space as the result 
of a direct assimilating and differentiating action exercised 
