DEVELOPMENT. 169 



those, reticulate thickenings. Here a number of cells unite 

 together to form a tube : and there they become almost solid 

 by the internal deposition of woody or other substance. 

 Through such changes, too numerous and involved to be here 

 detailed, the originally uniform cells go on diverging 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 described 

 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 consists only of partially-differentiated tissues. By 

 the diverse changes these gradually undergo they are trans- 

 formed into bones, muscles, blood-vessels, and nerves. The 

 extreme softness and delicacy of these primary tissues, 

 renders it difficult to trace the initial stages of the dif- 

 ferentiations. In consequence of the colour of their con- 

 tents, the blood-vessels are the first parts to become distinct. 

 Afterwards the cartilaginous parts, which are the bases of 

 the future bones, become marked out by the denser aggrega- 



