72 BASIC METHODS FOR EXPERIMENTS 



Cutting Serial Sections 



Much may be said in favor of the so-called precision (sliding) 

 microtome, but where many serial sections are demanded as in 

 the case of several thousand minute eggs, an automatic rotary 

 microtome may be employed to advantage. First of all, one 

 saves much time by using such. Also, it gives better ribbons 

 and minimizes the chance of losing sections. And if the instru- 

 ment is a high-grade one, properly cared for, it will cut sections 

 of uniform thickness, without skipping. I recommend the 

 Spencer Automatic Rotary Microtome. With three different 

 microtomes of this type I have cut perfect serial sections of three 

 or four microns without any fear that sections jam, fail to ribbon 

 or come oflF double the thickness for which the instrument is 

 set. Doubtless many another high-grade automatic rotary is 

 capable of equal performance. 



The disc is mounted on to the microtome with that flat sur- 

 face of the paraffin block nearer which is the patch of eggs downwards. 

 In this wise, the knife cuts first through the thin layer of paraffin 

 underlying the eggs, then through the eggs and last through 

 the thicker paraffin layer above the eggs. If the block is 

 mounted the other way around, the sections do not ribbon 

 properly. Once the knife is in place, the block is adjusted by 

 means of the screws of the disc-holder so that the lower edge of 

 the block is parallel to the edge of the knife. The block is 

 now most firmly clamped in place. 



For serial sections of eggs, each of which is a composite of 

 oil, yolk and granules of different physico-chemical make-up, 

 in a more fluid menstruum, so fixed that not only the nucleus 

 with its components and the cytoplasm are preserved but also 

 the various cytoplasmic inclusions, it is absolutely mandatory 

 to use the best microtome knife available. Such substitutes as 

 safety razor blades are interdicted; they merely tear through 

 the eggs, dislodging the oil-drops and yolk-spheres now of a 

 stone hardness due to fixation, dehydration and clearing. 

 Indeed, such thin blades often dislodge chromosomes and sper- 

 matozoa attached to the eggs. These imitation microtome 

 knives do very well for objects so fixed that they are vacuolated 

 masses, the oil, yolk and granules having been destroyed; but 



