146 Mr. P. A. Buxton on the 



this for some days, well washed in 70 % and 90 % alcohol, 

 and then embedded. The chitin is certainly softened by 

 this solution, but will regain some of its hardness if it is 

 simply embedded in hot paraffin. It appears that heat, 

 absolute alcohol, and xylol, all exercise a marked harden- 

 ing effect on chitin. It is best, then, that the head or 

 insect should be dehydrated as much as possible in 90 % 

 alcohol, left a short time in absolute alcohol, and cleared in 

 chloroform. I then place it for some days in a chloroform 

 solution of paraffin, and finally drop it into the hot paraffin 

 of the ordinary embedding bath. Here it remaiiis only 

 long enough for it to attain the temperature of the bath, 

 and is then removed in the crucible or other vessel in which 

 the paraffin is contained, and placed under a vacuum pump ; 

 the pump will quickly remove the chloroform, most of 

 which has by now become diffused into the paraffin. The 

 mass may then be turned out into a mould and cooled. 

 Terpineol has also been used as a softening reagent and 

 it appears quite satisfactory, though I have not much 

 experience of it. 



Celloidin. — I have used this to some extent, though 

 I no longer do so, because I find it unnecessary if spirit 

 soap is used as described above. It cannot be relied upon 

 to penetrate a whole insect unless thin celloidin be employed 

 for many days. 



Both with and without celloidin I have been able to 

 obtain serial sections of the head of Micropteryx of con- 

 siderable thinness. I have several series of 3'5 ij., which is 

 not by any means too thin, because of the smallness and 

 complex structure of the brain. 



Practical experience teaches me that it is never safe to 

 move either complete brains or sections from absolute 

 alcohol to xylol or vice versa, but that an intermediate 

 mixture of the fluids should always be employed. Unless 

 this is done the ganglion cells will frequently break away 

 from the axonic part of the brain. 



III. Staining. 



For general study sections should be stained with Dela- 

 field's HvEmatoxylin, and orange G (eosin may also 

 be used, but I think that the orange G gives better results). 

 Such sections are excellent for preliminary work, and I 

 always use this stain as a standard test for a fixative which 

 is new to me. 



