Cells Partially Differentiated Can Be Altered 27 
equal and consequently large cells, which divide again 
later in their turn each into two correspondingly smaller 
cells and so on; but on the contrary the breaking up is 
from the first into small cells.” 
It is upon these indifferent cells that the half of the 
egg already developed exercises its formative action. 
These nuclei which arise from cells of the half of the 
embryo already developed must nevertheless possess very 
definite specific properties; some come from ectodermic 
cells, others from mesodermic and others from entodermic 
cells. And if the medullary plate, the notochord, etc., 
have already been formed, the vagrant nuclei come also 
from cells which are in an advanced stage of development. 
And yet when they have once emigrated and have become 
scattered through the yolk of the injured side, they 
remain no less indifferent in relation to the formative 
stimuli which come off later from the part already 
formed, than in the cases where they arise entirely from 
the injured half of the egg. From whatever cells of the 
embryonal half already developed they may have been 
produced, they are capable of any somatization whatever, 
for this depends only on the place at which they happen 
to stop or become arrested during their migration into 
the yolk plasma of the injured half of the egg. 
The same thing can take place in the blastomeric 
nuclei also as soon as they once find themselves outside 
the group, which, according to the hypothesis above 
stated, would form the central zone of development: in 
relation to the ontogenetic stimuli which from now on 
TWilhelm Roux: Uber das entwicklungsmechanische Vermogen 
jeder der beiden ersten Furchungszellen des Eies. Verhandlungen 
der anat. Ges. Wien. June 1892. P. 34, 35. Gesamm. Abhandl. 
Zw. Bd. P. 782, 783. 
