324 



THE MICROSCOPE AND ITS REVELATIONS. 



larger concretions as are shown at a, a, are gradually produced. The 

 structure of these, especially when examined by Polarized light, is found 

 to correspond very closely with that of the small calculous concretions 

 which are common in the urine of the Horse, and which were at one time 

 supposed to have a matrix of cellular structure. The small calcareous 

 concretions termed ' otolitlis? or ear-stones, found in the auditory sacs of 

 Fishes, present an arrangement of their particles essentially the same. 

 Similar concretionary spheroids have already been mentioned ( 613) as 

 occurring in the skin of the Shrimp and other imperfectly-calcified shells 

 of Crustacea; they occur also in certain imperfect layers of the shells of 

 Mollusca; and we have a very good example of them in the outer layer of 

 the envelope of what is commonly known as a 'soft egg/ or an 'egg 

 without shell,' the calcareous deposit in the fibrous matting already 

 described ( 668) being here insufficient to solidify it. In the external 

 layer of an ordinary egg-shell, on the other hand, the concretions have 

 enlarged themselves by the progressive accretion of calcareous particles, 

 so as to form a continuous layer, which consists of a series of polygonal 

 plates resembling those of a tessellated pavement. In the solid ' shells > 



Fio. 499. 



Artificial Concretions of Carbonate of Lime. 



of the eggs of the Ostrich and Cassowary, this concretionary layer is of 

 considerable thickness; and vertical as well as horizontal sections of it 

 are very interesting objects, showing also beautiful effects of color under 

 Polarized light. And from the researches of Prof. W. C. Williamson on 

 the scales of Fishes ( 657), there can be no doubt that much of the cal- 

 careous deposit which they contain is formed upon the same plan. 



713. This line of inquiry has been contemporaneously pursued by 

 Prof. Harting, of Utrecht, who, working on a plan fundamentally the 

 same as that of Mr. Eainey (viz., the show precipitation of insoluble salts 

 of Lime in the presence of an Organic * colloid'), has not only confirmed 

 but greatly extended his results; showing that with animal colloids (such 

 as egg-albumen, blood-serum, or a solution of gelatine) a much greater 

 variety of forms may be thus produced, many of them having a strong 

 resemblance to Calcareous structures hitherto known only as occurring 

 in the bodies of Animals of various classes. The mode of experimenting 

 usually followed by Prof. Harting, was to cover the hollow of an ordi- 

 nary porcelain plate with a layer of the organic liquid, to the depth of 

 from 0.4 to 0.6 of an inch; and then to immerse in the border of the 



