MODE OF PREPARING THIN SECTIONS. 565 



destruction of the specimen. In ' mounting ' the specimen, liquid 

 Balsam should be employed, and only a very gentle heat (not 

 sufficient to produce air-bubbles, or to loosen the specimen from 

 the glass) should be applied ; and if after it has been mounted the 

 section should be found too thick, it will be easy to remove the 

 glass cover and to reduce it further, care being taken to harden to 

 the proper degree the Balsam which has been newly laid-on. 



437. If a number of Sections are to be prepared at once (which 

 it is often useful to do for the sake of economy of time, or in 

 order to compare sections taken from different parts of the same 

 Spine), this may be most readily accomplished by laying them 

 down, when cut-off by the saw, without any preliminary prepara- 

 tion save the blowing of the calcareous dust from their surfaces, 

 upon a thick slip of glass well covered with hardened Balsam ; a 

 large proportion of its surface may thus be occupied by the Sections 

 attached to it, the chief precaution required being that all the 

 sections come into equally close contact with it. Their surfaces 

 may then be brought to an exact level, by rubbing them down, 

 first upon a flat piece of Grit (which is very suitable for the rough 

 grinding of such sections), and then upon a large Water-of-Ayr 

 stone whose surface is ' true. ' "When this level has been attained, 

 the ground surface is to be well washed and dried, and some 

 Balsam previously hardened is to be spread over it, so as to be 

 sucked-in by the sections, a moderate heat being at the same time 

 applied to the Glass slide ; and when this has been increased suffi- 

 ciently to loosen the sections without overheating the Balsam, the 

 sections are to be turned-over, one by one, so that the ground sur- 

 faces are now to be attached to the Glass slip, special care being 

 taken to press them all into close contact with it. They are then 

 to be very carefully rubbed-down, until they are nearly reduced to 

 the required thinness; and if, on examining them from time to 

 time, their thinness should be found to be uniform throughout, the 

 reduction of the entire set may be completed at once ; and when 

 it has been carried sufficiently far, the sections, loosened by 

 warmth, are to be taken-up upon a camel-hair brush, dipped in 

 Turpentine, and transferred to separate slips of Glass whereon some 

 liquid Balsam has been previously laid, in which they are to be 

 mounted in the usual manner. It more frequently happens, how- 

 ever, that, notwithstanding every care, the Sections, when ground 

 in a number together, are not of uniform thickness, owing to some 

 of them being underlaid by a thicker stratum of Balsam than 

 others are ; and it is then necessary to transfer them to separate 

 slips before the reducing process is completed, attaching them with 

 hardened Balsam, and finishing each Section separately. 



438. A very curious internal Skeleton, formed of detached 

 Plates or Spicules, is found in many members of this Class ; often 

 forming an investment like a Coat of Mail to some of the viscera, 

 especially to the Ovaries. The forms of these plates and spicules 



