68 C. M. CHILD. 



The process is briefly as follows : After exposure of a cut sur- 

 face some slight proliferation occurs which results in healing 

 unless another cut surface be so near that the cells arising from 



o 



both are in contact ; if this is the case then organic union be- 

 tween the two cut surfaces is rapidly established. In the closure 

 of the ends of cylindrical pieces this process is usually completed 

 in a few days, but in certain other cases it may proceed much 

 more slowly. For example, in pieces which are split down one 

 side (Fig. 10) and in which both of the longitudinal cut margins 

 roll inward as in Fig. 11, the process of closure often requires 

 two months or more for completion. In such cases the longitu- 

 dinal cut margins usually roll inward so far that they are not in 

 contact. At one or both of the ends, however, the closure may 

 occur in nearly the typical manner. From the end the new 

 tissue begins to grow along the longitudinal cut, and as it grows 

 actually draws the cut edges together to a certain extent. The 

 process may be compared for the sake of illustration to that of 

 sewing up the longitudinal slit in the piece from one or both ends. 

 If we take, for example, a case where two cut surfaces are in con- 

 tact at one point and diverge at an acute angle from this point, 

 we find that the growth of new tissue always begins at the point 

 of contact. From this point growth and the formation of a thin 

 membrane continue for a certain distance along the diverging 

 cut surfaces, the extent of the membrane depending in a given 

 species on the angle of divergence of the surfaces. This thin 

 membrane is itself somewhat elastic and so tends to approximate 

 the cut surfaces in greater or less degree unless opposed by other 

 conditions. The approximation of the surfaces renders possible 

 a further extension of the membrane between them, and so the 

 process continues unless at some point the cut surfaces are so 

 situated that the elasticity of the new tissues is insufficient to bring 

 them into contact, or near enough to permit the extension of the 

 thin membrane between them. In such a case the process of 

 closure must cease, as often occurs. That this is actually what 

 occurs I have convinced myself by repeated examination of speci- 

 mens cut in such manner that at least some parts of the cut sur- 

 faces were not in contact while others were. The diagrammatic 

 Figs. 2630 will serve to illustrate the process. Fig. 26 shows 



