. The microscope and its revelations. r only one is produced. An examination of tig. 699 willclearly show the mode in which the operation is effected. This figurerepresents a section of one of the valves of Unio occiclens, taken per-pendicularly to its surface, and passing from the margin or lip (atthe left hand of the figure) towards the hinge (which would be atsome distance beyond the right). This section brings into view thetwo substances of which the shell is composed, traversing the outeror prismatic layer in the direction of the length of its prisms, andpassing through the nacreous linin
. The microscope and its revelations. r only one is produced. An examination of tig. 699 willclearly show the mode in which the operation is effected. This figurerepresents a section of one of the valves of Unio occiclens, taken per-pendicularly to its surface, and passing from the margin or lip (atthe left hand of the figure) towards the hinge (which would be atsome distance beyond the right). This section brings into view thetwo substances of which the shell is composed, traversing the outeror prismatic layer in the direction of the length of its prisms, andpassing through the nacreous lining in such a manner as to bringinto view its numerous lamina?, separated by the lines b, c c,Arc. These lines evidently indicate the successive formations of thislayer, and it may be easily shown by tracing them towards thehinge on the one side and towards the margin on the other, that atevery enlargement of the shell its whole interior is lined by a newTnacreous lamina in immediate contact with that which preceded FIG. 699.—Vertical section of the lip of one of the valves of theshell of Unio : n, b, c, successive formations of the outerprismatic layer ; a, b, c, the same of the inner nacreous layer. The number of such lamina}, therefore, in the oldest part of the shellindicates the number of enlargements which it has undergone. Theouter or prismatic layer of the growing shell, on the other hand, isonly formed where the new structure projects beyond the margin ofthe old ; and thus we do not find one layer of it overlapping anotherexcept at the lines of junction of two distinct formations. When theshell has attained its full dimensions, ho\\e\er. new laminae of bothlayers still continue to be added, and thus the lip becomes thickenedl>y successive formations of prismatic structure, each being appliedto the inner surface of the preceding, instead of to its free like arrangement may be well seen in the Oyster, with this differ-ence, that the successive layers hav
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
