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 



