A GUIDE FOR STUDENTS OF MICROTOMY 725 



add about the same quantity of xylol or cedar oil. Shake, leave half an 

 hour, and then transfer the material to pure xylol or cedarwood oil ; leave 

 half an hour. I'our away some of the xylol, either add chips of hard 

 wax to cover the tissue, or add some of the stock xylol and wax mixture. 

 Leave an hour in thermostat on the upper shelf, pour off, and add 

 molten pure wax ; leave one or two hours on the bottom shelf. Imbed 

 blocks (§§ 156, 157). 



1422. Example III. Preparation of an Embryo for Serial Sections. 

 Fix in Bouin's fluid, corrosive acetic or picro-nitrie, overnight (§§ 115, 

 68, 102). In the case of the first and last mentioned fixatives, the 

 embryo is afterwards transferred to 30 per cent, alcohol (half-hour), 

 50 per cent, (two hours), and then washed for a day in several changes 

 of 70 per cent. The corrosive acetic fixed specimens are similarly 

 treated except that at this stage iodine solution is added to the 70 per 

 cent, (or this may be done in 90 per cent.) alcohol till the corrosive 

 sublimate is removed. Leave overnight in 90 per cent, alcohol (or 

 at least three or four hours), and at least six hours in two changes of 

 absolute alcohol (preferably overnight). De-aleoholisation and clearing 

 must be done carefully as directed in § 134, p. 68. It is a good plan 

 to bring embryos from absolute alcohol, through several gradually 

 strengthening mixtures of alcohol and cedar-wood oil — to pure cedar- 

 wood oil, and then wash out in benzol. Imbed in wax as described in 

 § 158, generally about one hour in benzol and wax, and two hours in pure 

 wax. Imbed blocks (§§ 156, 157). Now read §§ 174 et seq. 



1423. General Rules and Hints for Students. (1) Keep all your 

 bottles and capsules as clean as possible. 



(2) Try to keep your bench in order. 



(3) Keep notes of the time necessary for changing reagents. 



(4) Thoroughly clean your slides and coverslips in acid alcohol before 

 using. See addendum. 



(5) Note that corrosive sublimate tends to harden material. 



(6) Corrosive sublimate is difficult to remove from tissue unless you 

 use iodine. If not properly removed you will find numerous pin-shaped 

 crystals in the finished sections. § 68. 



(7) Corrosive sublimate attacks the surface of steel and other metals. 

 Use quills or wooden needles for manipulating tissue in sublimate. 



(8) Watery stains after picric acid fixation will cause maceration if 

 prolonged. § 98. 



(9) Unless very well washed out, picric acid should not be used in 

 conjunction with thionin or toluidin blue. Precipitates form. Certain 

 other dyes do likewise. 



(10) Osmic acid crystals should be dissolved in the purest distilled 

 water. Wash the tube with distilled water before you break it, remov- 

 ing label. Wash out capsules and bottles for osmic acid solutions in 

 distilled water. Keep solutions in shade or dark. § 40. 



(11) Osmic acid tends to harden yolk and certain other cell materials. 

 The vapour of osmic acid is injurious to the eyes and nose. 



(12) Osmic acid and fixatives containing it inhibit staining, but if 

 necessary you can induce osmicated material to stain in delicate dyes 

 by bringing sections down to distilled water and treating in a 0-25 per 

 cent, solution of permanganate of potash for a short time. Perman- 

 ganate also decolourises sections. See p. 309 and § 701. 



(13) Nitric acid tends to soften ehitin and yolk, but it may inhibit 

 staining a little. § 102. 



(14) Imbed material in paraffin in the shortest time possible, for 

 materials left in the thermostat longer than necessary go hard, especially 

 from xylol ; this refers especially to vertebrate material and yolky 

 embryos. 



