4 FORMATION OF SHELLS OF ANIMALS, ETC., 
of animal tissues, it is indispensably necessary that the 
parts examined should be in the earliest stages of the 
process, and before the calcifyimg membrane is entirely 
covered with the globular coalescing deposit. The usual 
plan of examining shells in thin vertical sections is entirely 
useless, unless it be simply to see the number and arrange- 
ment of their layers; the part of the section in such 
specimens, in which the calcifying process ought to be 
best seen, being entirely ground off. This part, being 
the softest, can only be preserved in the process of grind- 
ing by extreme care, and by keeping the lower edge of the 
section always thicker than the upper. 
Now, as the perfect resemblance of the globular form 
of the carbonate of lime, as prepared artificially, and as 
occurring in nature, indicates a corresponding similarity 
in the nature of the process by which they are formed, 
and an identity of the forces concerned in their formation, 
the careful experimental investigation of the precise mode 
of formation of the artificial products cannot fail to throw 
great light upon the genesis of the natural ones, and thus 
tend to emancipate this department of histology from the 
obscurity in which it now hes, and bring it under the 
domain of experimental physical science. Besides, I may 
add that the artificial calculi, when carefully and properly 
prepared, present the microscopist with a new class of 
polariscope objects not exceeded in beauty or brilliancy 
by any others. But even these optical phenomena, and 
the physical facts connected with them, may, when more 
minutely investigated, be found to possess other points of 
interest besides their mere appearance ; for, occurring as 
they do in bodies whose molecular constitution and mode 
of formation can be demonstrated, they ought to open 
new avenues to a more complete understanding of the 
