CHAPTER XXXVIII. 



A GUIDE FOR STUD.ENTS OF MICROTOMY. 



1046. Three Examples for Beginners : (1) The preparation of whole 

 stained mounts of some small object (Daphnids). 



(2) The preparation of sections of the muscle or an organ of a 

 vertebrate. 



(3) The preparation of an embryo (or tadpole) for the making of serial 

 sections. 



Example I. From a pond or ditch obtain some water-fleas (Daphnia 

 or Simocephalus) ; allow the jar to stand for several hours till the 

 suspended material has settled. Capture some of the organisms as 

 follows : Take a piece of glass tubing some 8 inches in length ; place 

 a finger over one end, dip the other end under the water and by taking 

 away the finger, suck up some of the Daphnids into the tube ; put your 

 finger over the end of the tube, remove the latter and transfer the 

 organisms to a capsule or watch-glass about 2 inches in diameter. With 

 a clean pipette carefully suck up most of the water, hardly allowing the 

 animals enough to swim in^; now add a fixative to kill the organisms 

 (see 2), and to coagulate their protoplasm ( 29) as rapidly as possible 

 so as to leave the groups of cells forming the organs intact and 

 in situ. 



Use corrosive acetic acid ( 63), 2 per cent, acetic acid in saturated 

 aqueous corrosive. Pour the fixative into the watch-glass or capsule, 

 till it is full (the watch-glass or capsule contains about 15 to 20 c.c,)- 

 Place a glass square or plate over the capsule, and leave it for thirty 

 minutes. The organisms become opaque, indicating the coagulation of 

 the proteids of their cells. 



With a pipette carefully remove as much of the fixing fluid as possible. 

 Now that the organisms are killed, the mercury salt must be removed ; 

 unless the fixative is thoroughly removed, it will form masses of pin-shaped 

 crystals at a later stage when the animals are being mounted in 

 balsam. 



To remove the corrosive sublimate, it is necessary to convert it into 

 another substance which may be more easily washed away ; this is 

 effected by immersing the animals in some 70 per cent, alcohol which 

 has been coloured light port -wine shade with tincture of iodine ( 63), 

 whereupon the mercury bichloride becomes mercury iodide, which is 

 very soluble in 70 per cent, alcohol. The iodine and alcohol mixture 



