140 THE INDUCTIONS OF BIOLOGY. 



in their interiors ; and those, net- works of fibres. Here a 

 number of cells unite together to form a tube; and there 

 they become solid by the internal deposition of woody or other 

 matter. Through such changes, too numerous and involved 

 to be here detailed, the originally uniform cells go on diverg- 

 ing and re-diverging, until there are produced various forms 

 that seem to have very little in common. 



The arm of a man makes its first appearance in as simple 

 a way as does the shoot of a plant. According to Bischoff, it 

 buds-out from the side of the embryo, as a little tongue-shaped 

 projection, presenting no differences of parts ; and it might 

 serve for the rudiment of some one of the various other organs 

 that also arise as buds. Continuing to lengthen, it presently 

 becomes somewhat enlarged at its end; and is then desci'ibed as 

 a pedicle bearing a flattened, round-edged lump. This lump is 

 the representative of the future hand ; and the pedicle, of the 

 future arm. By and by, at the edges of this flattened lump, 

 there appear four clefts, dividing from each other the buds of 

 the future fingers ; and the hand as a whole grows a little 

 more distinguishable from the arm. Up to this time, the 

 pedicle has remained one continuous piece ; but it now begins 

 to show a bend at its centre, which indicates the division into 

 arm and forearm. The distinctions thus rudely indicated, 

 gradually increase : the fingers elongate and become jointed ; 

 and the proportions of all the parts, originally very un- 

 like those of the complete limb, slowly approximate to 

 them. During its bud-like stage, the rudimentary 



arm is nothing but a homogeneous mass of simple cells, with- 

 out any arrangement. By the diverse changes they gradually 

 undergo, these cells are transformed into bones, muscles, 

 blood-vessels, and nerves. The extreme softness and delicacy 

 of this primary cellular tissue, renders it difficult to trace the 

 initial stages of these differentiations. In consequence of the 

 colour of their contents, the blood-vessels are the first parts to 

 become visible. Afterwards the cartilaginous parts, which 

 are the bases of the future bones, become marked out by the 



