66 RALPH S. LILLIE. 



the conditions may be limited; and in fact most individual or- 

 ganisms and cells have definite limits of size, a balance being 

 eventually reached where metabolic destruction balances con- 

 struction; nevertheless the construction of new living substance, 

 the essential process underlying growth, continues in all cases 

 throughout life, and when it ceases, life ceases; the constructive 

 process cannot suffer permanent interruption. Even where the 

 organism as a whole seems to have reached its final dimensions 

 there are always special cells or regions which when placed under 

 appropriate conditions still show indefinite proliferative activity. 

 In higher organisms such regions are represented more especially 

 by the germinal epithelium ; and a single detached cell from this 

 region exhibits under certain conditions that unique property of 

 definite and orderly proliferation known as development, which 

 leads to the production of an organism similar in its minutest 

 details to that of which the cell originally formed part. This is 

 the chief form which the process of reproduction takes in higher 

 animals, and such a manner of consideration shows that no funda- 

 mental distinction can be drawn between growth and repro- 

 duction. In many organisms almost any portion of sufficient 

 size which is detached from the whole whether by some phy- 

 siologically normal mechanism like fission or as the result of 

 operation may continue its growth after isolation, redifferen- 

 tiate, and eventually regain the form and physiological characters 

 of the original stock. Whole plants may thus be reproduced from 

 artificial cuttings, and the same is true of many lower animals 

 (protozoa, hydroids, planarians). In such cases the distinction 

 between growth and reproduction becomes ill-defined, and repro- 

 duction appears as essentially a form of discontinuous growth; 

 and in an organism like yeast, where the growing cell-masses 

 may either cohere in chains or fall apart into separate cells 

 apparently as the result of purely casual conditions the dis- 

 tinction ceases to have more than a formal significance. If the 

 cell-chain is regarded as the unit organism, increase in its length 

 by budding is a matter of growth, if the single cell is so regarded, 

 the same phenomenon becomes an instance of reproduction. 



It is important for the purpose of the present discussion that 

 the artificiality of this distinction should be recognized at the 



