THE LAW OF EVOLUTION CONTINUED. 345 



proportioned in different localities; and multiplication of 

 isomeric forms leads to further mixtures and combinations 

 that constitute many minor distinctions of parts. Here a 

 mass darkened by accumulation of hematine, presently dis- 

 solves into blood. There fatty and albuminous matters 

 uniting, compose nerve-tissue. At this spot the nitrogenous 

 substance takes on the character of cartilage; and at that, 

 calcareous salts, gathering together in the cartilage, lay the 

 foundation of bone. All these chemical differentiations 

 sloAvly and insensibly become more marked and more mul- 

 tiplied. 



Simultaneously there arise contrasts of minute struc- 

 ture. Distinct tissues take the place of matter that had 

 previously no recognizable unlikenesses of parts; and each 

 of the tissues first produced undergoes secondary modifi- 

 cations, causing sub-species of tissues. The 

 granular protoplasm of the vegetal germ, equally with that 

 which forms the unfolding point of every shoot, gives origin 

 to cells that are at first alike. Some of these, as they grow, 

 flatten and unite by their edges to form the outer layer. 

 Others elongate greatly, and at the same time join together 

 in bundles to lay the foundation of woody-fibre. Before 

 they begin to elongate, certain of these cells show a break - 

 ing-up of the lining deposit, which, during elongation, be- 

 comes a spiral thread, or a reticulated framework, or a 

 series of rings; and by the longitudinal union of cells so 

 lined, vessels are formed. Meanwhile each of these dif- 

 ferentiated tissues is re-differentiated: instance that which 

 constitutes the essential part of the leaf, the upper stratum 

 of which is composed of chlorophyll-cells that re- 

 main closely packed, while the lower stratum becomes 

 spongy. Of the same general character are the 

 transformations undergone by the fertilized ovum, which, 

 at first a cluster of similar cells quickly reaches a stage in 

 which these cells have become dissimilar. More frequently 

 recurring fission of the superficial cells, a resulting smaller 

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