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after a process which offers so ruany illustratious iu the physiology of higher 

 animal life. 



The President then invited discussion. 



Mr. Charles Stewart said he did not understand that the President offered . 

 any explanation of the way in which absorption took place. The dentine of 

 the tooth was removed under the influence of the membrane in contact with 

 it, but he did not say how, and he left the question, strictly speaking, without 

 an answer. The facts of the case had been quite truly given by Mr. Waller. 

 We had, in the case of the limestone, and also of the calcareous shells, cer- 

 tain cavities which were more or less occupied by the sponge, and when 

 examined under the microscope, they were found very constant in the cha- 

 racter of their surface. From the main cavities there were, however, certain 

 minute processes which tapered off into what were practically sharp points, 

 these were occupied by the soft substance of the sponge. His contention was 

 that no Annelid was likely to make so fine a passage as this, going off prac- 

 tically to a point. Of course Annelids did bore with facility into limestone 

 and shells, and even into harder substances. Very much stress was laid on 

 the depressions being segments of spheres ; that was a very important 

 point which we should recognise, but he did not think it was quite worked 

 out to a natural conclusion by Mr. Waller. It was perfectly certain that a 

 homogeneous body might be worked to that form. Either mechanical or 

 chemical action commencing erosion from a point would make a regular 

 concave depression. Or, if the erosion be equal over the surface attacked, 

 but the resistance less at certain points, a similar pitted surface would be 

 produced. If we examined a piece of iridescent glass under the 

 microscope, the surface would present a number of regular depressions 

 exactly like those said to be made by the Annelids. Mr. Waller would 

 not allege these were excavated by Annelids. In the glass there were 

 certain points and streaks where oxide of iron was more abundant, and 

 around these points as centres decomposition spread into layers and 

 laminae, which shelled off from the surface of the glass, leaving curvilinear 

 cavities as seen in the rocks and shells. Mr. Waller seemed to think that 

 the analogy was very strained between erosion of bone which occurred in a 

 natural process, and that which occurred in the case of the sponge. He 

 failed to see that the analogy was strained. What we had to deal with in 

 the case of bone or feeth was a mixture of organic matter with certain 

 lime salts, and we had, in the case of the shell, a mixture of inorganic 

 matter and lime salts. Now, the actual surface of the bone was covered with 

 certain cells which presented no great difference from the cells in contact with 

 the shell bored by the sponge. Those cells would eat out the bone, forming 

 a surface so closely resembling the chambers containing the sponge that he 

 did not think they could be distinguished from one another. He thought 

 the cells and the substances were very similar, and the effects produced 

 identical in character. 



Mr. M. Hawkins Johnson said he had no theory of his own to advance on the 

 subject, but he did not place implicit confidence in those he had heard. 

 He once saturated an oyster-shell, which was full of these holes, with melted 



