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 



7 Wilhelm 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. 



