48 SENESCENCE AND REJUVENESCENCE 



other kind, which are not necessarily qualitative in any sense. 

 And, finally, we are able to determine experimentally the devel- 

 opment of very different morphological characters by changes in 

 conditions which affect primarily the rate and not the character 

 of the metabolic reactions (Child, 'n). To what extent quantita- 

 tive differences in the dynamic processes actually serve as a basis 

 for specialization and differentiation we do not know, although it 

 is certain that they are a much more important factor than most 

 biologists have been accustomed to believe. 



But, supposing that quantitative or qualitative differences 

 arise or exist in different regions of the developing organism, how 

 can they persist in a substance of the physical consistency of pro- 

 toplasm ? It is here that the colloid condition of the substratum 

 plays a very important part. The organic colloids with their 

 slight diffusibility, their effect on the diffusion of other substances, 

 their viscosity and differences of aggregate condition, afford possi- 

 bilities for the localization as well as the origination of different 

 processes which do not exist in any other known medium. The 

 experiments on the production of form and structure by means of 

 chemical reactions in a colloid substratum outside the organism 

 demonstrate how readily even complex morphological features 

 may arise under such conditions, and in such cases we are often 

 able to analyze the process of differentiation. We have then in the 

 colloid substratum a real basis for differentiation, and the problem 

 of morphogenesis becomes accessible to scientific investigation and 

 analysis, instead of being merely restated in terms of some "vital- 

 istic" principle or of determinants or other ultimate units. 



The embryonic or undifferentiated cell is distinguishable from 

 the specialized or differentiated cell rather by the absence than by 

 the presence of definite morphological features. It represents the 

 cell of the species reduced to its simplest morphological terms, 

 consisting essentially of nucleus and relatively homogenous cyto- 

 plasm. 1 It is of course true that cells which are not morphologically 



1 Embryonic cells are shown in Fig. 113 (p. 285), and in the smaller cells of Fig. 

 187 (p. 347), and in Fig. 194, em (p. 348). Cells which are embryonic in appear- 

 ance are represented more or less diagrammatically in various other figures, e.g., 

 Figs. 71-74 (pp. 206, 208) and Fig. 192, pc (p. 348). 



