DEVELOPMENT. 1 4 1 



denser aggregation of their constituent cells, and the produc- 

 tion, between these of a hyaline substance which unites them 

 into a translucent mass. When first perceptible, the muscles 

 are gelatinous, pale, yellowish, transparent, and indistinguish- 

 able from their tendons. The various other tissues of which, 

 the arm consists, beginning with very faintly-marked differ- 

 ences, become day by day more definite in their outlines and 

 appearances. In like manner, the units composing 



these tissues, severally assume increasingly-specific characters. 

 The fibres of muscle, at first made visible in the midst of 

 their gelatinous matrix only by immersion in alcohol, grow 

 more numerous and distinct ; and by and by they begin to 

 exhibit transverse stripes. The bone-cells put on by degrees 

 tneir curious structure of branching canals. And so in their 

 respective ways with the units of skin and. the rest. 



Thus in each of the organic sub-kingdoms, we see this 

 change from an incoherent, indefinite homogeneity, to a 

 coherent, definite heterogeneity, illustrated in a quadruple 

 way. The originally -like units or cells, become unlike in 

 various ways, and in ways more numerous and marked as the 

 development goes on. The several tissues which these 

 several classes of cells form by aggregation, grow little by 

 little distinct from each other ; and little by little put on 

 those structural complexities, that arise from differentiations 

 among their component units. In the shoot, as in the limb, 

 the external form, originally very simple, and having much 

 in common with countless simple forms, organic and in- 

 organic, gradually acquires an increasing complexity, and an 

 increasing unlikeness to other forms. And meanwhile, the 

 remaining parts of the organism to which the shoot or limb 

 belongs, having been severally assuming structures divergent 

 from each other and from that of this particular shoot or 

 limb, there has arisen a greater heterogeneity in the organ- 

 ism as a whole. 



§ 52. One of the most remarkable inductions of embry- 



