246 PHYSIOLOGICAL DEVELOPMENT. 



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 interwoven hyphae obviously unlike the 

 mass of spores 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 Algce show just the same rela- 

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

 tubular branches of the thallus 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 continuous with one another. In Rivu- 

 laria, 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 Algce of all kinds repeat the antithesis. 

 In branched stems, when they consist of more than single 

 rows of cells, the outer cells become unlike the inner, as shown 

 in Fig. 35. Such types as Chrysymenia rosea show us this 



