GROWTH, DIFFERENTIATION, DEVELOPMENT. 511 



which, like masks, conceal the interior structure. These, in succession, 

 are cast off. 



After many such successive castings of the skin, the insect enters into 

 the pupa or chrysalis state. It has no organs of locomotion, and, as it 

 has been, with some degree of imagination, said, becomes an egg again. 

 After resting in this state for a certain time, it bursts its confinement, 

 and assumes the form of an aerial, swift-moving winged insect. This is 

 its imago state. 



It will now be convenient to give a more precise definition to terms 

 which have been hitherto used with a certain latitude. 



By the term growth is to be understood the increase in size of a struc- 

 ture, without its assuming any variation as respects the na- Definition of 

 ture of its fabric or of the functions it discharges. SS^ and 



By differentiation is meant an increase involving modifica- development, 

 tion of fabric and the assumption of new function. 



By development is meant a differentiation of a higher order, or com- 

 pound differentiation. Usually it implies growth and differentiation con- 

 jointly. 



As illustrations of the preceding definitions, it may be said that a crys- 

 tal grows, its enlargement presenting no structural variation and no new 

 quality. Cells differentiate from their normal spherical form, and, assum- 

 ing a cylindroid figure, give origin to vascular tissue, the vessels so aris- 

 ing serving for new purposes, as for the conveyance of gases or liquids. 

 A seed develops, for the organism to which it gives rise not only offers 

 continually increasing dimensions, but at all points the origination of 

 novel structures, arising by differentiation from adjacent and pre-existing 

 ones, these new structures having also new functions. 



By homogenesis is meant the production of an organism in all respects 

 like its parent ; by heterogenesis, the production of an or- Homoo . enesis 

 ganism unlike its parent. and heterogen- 



For the sake of brevity and simplicity, we may suppose esis ' 

 that there resides in every germ, and, therefore, in every organism, a prin- 

 ciple or quality which governs the collocation or grouping of new parts, 

 the same to which allusion has heretofore been made under the designa- 

 tion of plastic power. It is unnecessary for us here to burden our con- 

 ceptions of such a power with any hypotheses respecting its nature, it 

 being understood that we use the title of this supposed agent only as an 

 expression of convenience. 



The production of every organism appears, as far as existing observa- 

 tions and experiments go, to be referable to a previously ex- An organ ,- c 

 isting organism. This being admitted, generation and repro- molecule the 

 duction imply, as their starting-point, an organic molecule. pou 

 Such a combination, furnished with nutrition, grows, its plastic power 



