Differentiation of Neuroblasts 53 
The essential feature in the differentiation of any cell must be a 
change in its protoplasmic structure, and the possible factors 
involved in this change may be reduced to three, the protoplasm 
of the cell itself, the food supply, and chemical substances or 
physical forces coming either directly or indirectly from the 
medium immediately surrounding the cell. It is obvious that 
under the conditions of the experiment the food supply is not 
affected and is, therefore, the same for both the neuroblasts which 
differentiate and for those that do not, and that the change in the 
protoplasmic structure of the cell must be due to the way in which 
the food is assimilated. “The manner in which assimilation takes 
place is still hypothetical, but the most probable theory yet 
advanced is that the blood proteids are broken down by enzymes 
or other chemical substances and the active amino acids or other 
radicals then recombine after the pattern of the cells of the tissue 
involved. ‘That they recombine differently in different cells is 
due to the fact thar the blood proteids are broken down at differ- 
ent points, and this is in turn due to the presence of different kinds 
or amounts of enzymes or other activating chemical substances. 
When differentiation occurs, unless a vital force is postulated, it 
is necessary to assume that the kind or amount of enzymes must 
have been increased or decreased, or some other chemical sub- 
stance or substances have been added or subtracted. ‘The cell 
during its whole embryonic history has been repeating the same 
cycle of processes, namely, assimilating food in a definite way, 
increasing in size and dividing, and it is impossible to conceive that 
any tendency to develop in a certain direction, any adaptation to 
conditions, or any need of the organism can produce a new chemi- 
cal substance or inhibit the action of one already present. Differ- 
entiation of any cell must therefore occur because of a change in 
the chemical composition or physical properties of the lymph 
surrounding it. 
In case of the neuroblasts, the cells outside the medullary tube 
are also differentiating and the products of their metabolism must 
change, either in kind or amount, and these products must enter 
the lymph. It is therefore evident that the presence or absence 
of muscles in a given somite must influence the character of the 
