GENUS ORBITOLJTES: PHYSIOLOGY; REPARATION OF INJURIES. 209 



cells, took place, not from the broken edge, but from .the margin of the unbroken ; 

 just as, to use a professional simile, an ulcerated surface 'skins-over' by an extension 

 of the integument from its edges, not by the direct formation of skin upon the granu- 

 lation-surface itself. All the six rows subsequently produced, are conformable to 

 each other, and to the first or reparative row, from which they have obviously 

 extended themselves after the normal manner. It is observable, however, that the 

 breadth of these rows varies in different parts, being least where they invest the pro- 

 jecting portions of the fractured edge, and greatest where they sink into its hollows. 

 And thus it comes to pass, that the irregularities left in the shape of the disk, by the 

 loss of a large part of its substance, are gradually compensated, so as to restore it to 

 a form much more nearly corresponding to its typical symmetry. 



38. Even a very small fragment appears thus to serve as the nucleus for a new 

 disk. In fig. 8 is shown an example of this kind, in which the tendency to the repro- 

 duction of the typical form, by the compensative reparation just "described, is very 

 curiously marked. This specimen also presents the following very curious feature, 

 that the new growth has taken place from the inner margin of the original fragment 

 (aa), and not from its outer or growing margin, as in the cases previously noticed. 

 Having carefully examined it in various modes, I cannot entertain the slightest doubt 

 that such has been the case ; for the cells of the first new zone, as well as those of all 

 the zones subsequently produced, are so manifestly conformable to those of the 

 thinner and older portion of the fragment, and are so unconformable to those of the 

 thicker and newer margin, that it seems obvious that the sarcode must have extended 

 itself from the former part, along the fractured edge on each side, and have then 

 enveloped the margin which had been left entire. This may have more readily taken 

 place in the present instance, because at the part (aa) the fracture seems to have 

 followed the course of one of the zones, instead of passing, as at the sides of this 

 fragment, and in the instances previously cited, in such a direction as to cut the 

 zones transversely. 



39. The preceding instance clearly proves, that connexion with the central 

 nucleus is not in the least degree requisite for the continued growth of the peripheral 

 parts ; since these may be entirely detached from it, without any loss of vital activity. 

 The same inference may be deduced from the examination of specimens, in which, 

 the central portion of the disk having been broken-out, a growth of new zones seems 

 to have taken place from without inwards, so as to fill up the void space thus left. 

 In no other way can I account for the appearances presented by a specimen in my 

 possession, in which the included portion is as evidently unconformable to that 

 which surrounds it, as it is in the preceding case, but in which there is also an unfilled 

 void, the shape of one part of which clearly indicates that it occupies the site of the 

 original centre. The included portion, and not the peripheral, must therefore be the 

 after-growth in this instance ; and if a little more time had elapsed, the whole of the 

 central vacuity would probably have been filled up by it. 



