42O APPENDIX. 



In cutting celloidin or other specimens requiring to be flooded 

 while sectioned, the knife is likewise placed obliquely ; the sections 

 are removed with a soft wet brush and transferred to alcohol ; rolling 

 up of such sections does not occur. 



Cutting ribbon-series is a modification of the usual procedure, 

 and possesses great advantages where the possession of a complete 

 series of sections arranged in their natural sequence is important ; not 

 only for embryological studies, where it has become a necessity, but for 

 many other purposes, ribbon-cutting is to be preferred. The success 

 of the manipulation depends largely upon the proper consistence of 

 the paraffin, since the latter must be of just such hardness that while 

 firm enough to enable the sections as cut to push before them those 

 already in the chain, it must be sufficiently soft to enable the op- 

 posed edges of the sections to adhere together, and thus form the 

 " ribbon." Preparatory to cutting the paraffin block is trimmed as 

 accurately as possible into rectangular form, and so clamped in the 

 microtome that the longer sides of the rectangle are exactly parallel to 

 the edge of the transversely -set knife, the latter being placed at right 

 angles to its slide-ways. 



When the first section is cut it is not removed, but allowed to lie 

 upon the blade ; the knife being returned to its first position, the 

 tissue is raised the proper distance (generally .01 mm.), and a second 

 section is made, which, if the paraffin is of the proper character, 

 will adhere to the first, while the latter is pushed ahead for a distance 

 equal to the second section ; in this manner each section in turn drives 

 those previously cut before it, all adhering by their opposed edges 

 and forming a ribbon whose length is often limited only by the wishes 

 and the convenience of the worker. Care must be taken to keep the 

 cutting edge, especially its under surface, free from particles of par- 

 affin, since the presence of these will lead to furrows and cracks in 

 the sections. The sides of the block corresponding to the knife- 

 edge must also be kept exactly parallel, otherwise the ribbon will be 

 curved instead of straight. In case the paraffin in which the tissue 

 lies is too hard, the sections breaking apart instead of adhering, ele- 

 vating the temperature of the workroom or judiciously holding the 

 block in the vicinity of a flame for a short time will usually afford 

 relief; or the entire block may be coated with softer paraffin, which 

 is subsequently trimmed off from all but the two adhering sides. As 

 the ribbons are completed they are placed in covered trays upon 

 clean sized paper, protected from dust and high temperature. 



6. Fixing sections to the slide constitutes the next step after 

 cutting when tissues have been embedded in paraffin, whether pre- 



