•228 PHYSIOLOGICAL DEVELOPMENT. 



equilibration, there must arise this distinction between the 

 outer part adapted to meet outer forces, and the inner part 

 adapted to meet inner forces. And their respective actions, 

 as thus meeting outer and inner forces, must be what we call 

 their respective functions. 



§ 269. Aggregates of the second order exhibit parallel 

 traits, admitting of parallel interpretations. Integrated 

 masses of cells or units homologous with protophytes, 

 habitually show us contrasts between the characters of the 

 superficial tissues and the central tissues. Such among these 

 aggregates of the second order as have their component units 

 arranged into threads or laminae, single or double, cannot, of 

 course, furnish contrasts of this kind ; for all their units are 

 as much external as internal. We must turn to the more or 

 less massive forms. 



Of these, among Fungi, the common Puff-ball is a good 

 example — good because it presents this fundamental differen- 

 tiation but little complicated by others. In it we have a 

 cortical layer of cellular tissue obviously unlike the mass of 

 cellular tissue which it incloses. So far as the unlikeness 

 between external and internal parts is concerned, we see here 

 a relation analogous to that existing in the simple cell ; and 

 we see in it a similar meaning: there is a physiological 

 differentiation corresponding to the difference in the incidence 

 of forces. 



Under various forms the Algm show just the same rela- 

 tion. Where, as in Codium Bursa, we have ramified tubular 

 cells aggregated into a hollow globular mass, the outer and 

 inner surfaces are contrasted both in colour and structure; 

 though the tubules composing the two surfaces are con- 

 tinuous with one another. In Rivularia, again, we see the 

 like, both in the radial arrangement of the imbedded threads 

 and in the difference of colour between the exterior of the 

 imbedding jelly and its interior. The more-developed Algm 

 of all kinds repeat the antithesis. In branched stems, 



