766 Transactions of the Society. 



away by the dynamic influence of fresh nutritive matter, knowing, 

 so to speak, nothing of the past. The post is vacated by the 

 superannuated colloid, which first yields up its crystalloid associate 

 and then disappears, leaving the ground free for fresh organization. 



It may be observed that the parts which are thus swept away 

 are in the outcome, though possibly not in the beginning, the 

 circumferential portions of Haversian systems ; and that they are 

 removed in such a way as to include portions of two or three 

 adjacent systems. These are, of course, the oldest or first-formed 

 parts, of the several systems. The form assumed by the nutritive 

 matter which is presumably active in determining the breaking- 

 down of the old tissue is most interesting. At the points where 

 the bone is being broken down we find lodged in cup-shaped 

 recesses of eroded bone little masses of protoplasmic matter with ill- 

 defined surface but well-marked nuclei, which, to all appearance, 

 are the agents of the change. These are the so-called osteoclasts. 

 Their surface is ill-defined because, on the one hand, it merges into 

 the colloid of the vanishing bone, showing at the point of junction 

 a curious striate marking, suggestive of the existence of currents 

 running between the protoplasm and the old bone ; and on the 

 other, it is connected by processes with the protoplasmic material 

 filling the gradually increasing cavity, of which protoplasm it is in 

 fact an extension. It appears to me that we have here, before our 

 eyes, the direct application of a new and active colloid to an old 

 product of molecular coalescence, with the sequence of molecular 

 disintegration. 



I cannot resist the temptation of referring to one or two other 

 processes in which both molecular coalescence and molecular dis- 

 integration play a part. 



Mr. Kainey has shown that the first — molecular coalescence — 

 is an essential part of the building-up of the shells of the Mollusca. 

 The second — molecular disintegration — appears to me to come into 

 action in the later stages of the growth of molluscous shells. For 

 instance, in the whorled shells of the Gasteropods, apical parts of the 

 interior which have been occupied by the young mollusc are 

 removed as the growth of the animal and its shell proceeds, so as to 

 economize the space available in the whole shell. I have no know- 

 ledge of the existence of anything like osteoclasts in this case ; but 

 the contact of the tissues of a rapidly-growing mollusc with the old 

 coalescence formation will, I think, be sufficient to account for the 

 solution of hard structures now superannuated. 



Mr. George Busk, F.R.S., after seeing my letter in ' Nature ' 

 on this subject, was good enough to tell me that he saw in the prin- 

 ciples which I sought to establish an explanation of a phenomenon 

 which had puzzled him, namely the excavation of shell surfaces 

 to which Polyzoa bad been attached. He has lent me for ex- 



