170 THE INDUCTIONS OF BIOLOGY. 



tion of their constituent cells, and by the production between 

 these of a hyaline substance which unites them into a trans- 

 lucent mass. When first perceptible, the muscles are gela- 

 tinous, pale, yellowish, transparent, and indistinguishable 

 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 qualitative 

 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 

 their 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 called 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 simple forms in general, gradually acquires 

 an increasing complexity, and an increasing unlikeness to 

 other forms. Meanwhile, the remaining parts of the organism 

 to which the shoot or limb belongs, having been severally 

 assuming structures divergent from one another and from 

 that of this particular shoot or limb, there has arisen a 

 greater heterogeneity in the organism as a whole. 



52. One of the most remarkable inductions of embry- 

 ology comes next in order. And here we find illustrated 



