DIFFERENTIATION OF STRUCTURE. 21 



when the appendages and the segments into which they are 

 subdivided similarly become differentiated and coalesce. 



It is in virtue of such processes that the flowers of plants, 

 and the heads and limbs of the Arthropoda and of the Ver- 

 tebrata^ among animals, attain their extraordinary diversity 

 and complication of structure. A flower-bud is a segmented 

 body or axis, with a certain number of whorls of appendages ; 

 and the perfect flower is the result of the gradual differentia- 

 tion and confluence of these primitively similar segments and 

 their appendages. The head of an insect or of a crustacean 

 is, in like manner, composed of a number of segments, each 

 with its pair of appendages, which by differentiation and con- 

 fluence are converted into the feelers and variously modified 

 oral appendages of the adult. 



In some complex organisms, the process of differentiation 

 by which they pass from the condition of aggregated embryo 

 cells to the adult, can be traced back to the laws of growth 

 of the two or more cells into which the embryo cell is divided, 

 each of these cells giving rise to a particular portion of the 

 adult organism. Thus the fertilized embryo cell in the arche- 

 gonium of a fern divides into four cells, one of which gives 

 rise to the rhizome of the young fern, another to its first root- 

 let, while the other two are converted into a placenta-like 

 mass which remains imbedded in the prothallus. 



The structure of the stem of Chara depends upon the dif- 

 ferent properties of the cells, which are successively derived 

 by transverse division from the apical cell. An internodal 

 cell, which elongates greatly, and does not divide, is suc- 

 ceeded by a nodal cell, which elongates but little, and becomes 

 greatly subdivided ; this by another internodal cell, and so 

 on in regular alternation. In the same way the structure of 

 the stem, in all the higher plants, depends upon the laws 

 which govern the manner of division and of metamorphosis 

 of the apical cells, and of their continuation in the cambmni 

 layer. 



In all animals which consist of cell-aggregates, the cells 

 of which the embryo is at first composed arrange themselves 

 by the splitting, or by a process of invagination, of the blas- 

 toderm into two layers, the epihlast and the hypoblast^ be- 

 tween which a third intermediate layer, the mesoblast^ ap- 

 pears ; and each layer gives rise to a definite group of organs 

 in the adult. Thus, in the Vertebrata, the epiblast gives rise 

 to the cerebro-spinal axis, and to the epidermis and its deriva- 

 tives ; the hypoblast, to the epithelium of the alimentary 



