28 



larger objects should be given a proportionately longer time, a week 

 to a month, or even longer. 



If the object to be imbedded, such as many embryological speci- 

 mens, is one with large interior cavities with thin walls the transfer 

 from the thin solution to the thick solution may be attended by a 

 collapse of the walls and a consequent shriveling and distortion of 

 the specimen. Avoid this by allowing the thin solution to thicken 

 very gradually by evaporation in a dry atmosphere, as under a bell- 

 jar with calcium chlorid present until the solution has attained the 

 right consistency. To accomplish this it is only necessary to have 

 the cork of the vial containing the specimen perforated by a small 

 hole. A small piece of paper may be inserted with the cork, or with 

 porous corks no special effort need be made. Unless the thick solu- 

 tion has itself thickened by evaporation, with large specimens it is 

 advisable to follow the 6% bath with a stay in a thicker solution, as 

 10 or 12%, for a day or so. 



65. Imbedding. Pour off the 6% or 12% solution and add for 

 a short time at least a 12% solution of celloidin (in ether-alcohol). 

 The tissue is now ready for imbedding in 12%, which may be ac- 

 complished in either of two ways : (a) on a holder or (b) in a paper 

 box. Only those specimens need be imbedded in a box that, from 

 their shape, or for purposes of careful orientation or serial sectioning, 

 require a larger imbedding mass around them. 



(a) On a holder (wooden-block). Choose a block of a conveni- 

 ent size; put a drop or two of celloidin upon one end and insert a 

 pin vertically to the surface near the edge. Transfer the tissue from 

 the vial of thick celloidin to the block and lean it against the pin. 

 The shape of many tissues will obviate the need of a pin. Pour the 

 thick celloidin onto the tissue, drop by drop, moving the block in 

 such a way that the thick viscid mass may be made to surround and 

 envelop the tissue. Continue to add drops of celloidin at intervals 

 until the tissue is well surrounded, and then as soon as a slight film 

 hardens on the surface invert the holder bearing the tissue in a 

 shell-vial of large diameter or glass box containing enough chloroform 

 to cover the specimen. Cork or cover so that the chloroform will 

 not evaporate. If the piece of tissue is of awkward size and shape, 

 oiled paper may be wound around the end of the wooden holder and 

 tightly tied, the projecting hollow cylinder being long enough to 

 receive the object. The tissue may be put into the cylinder as before, 

 the celloidin slowly poured in drop by drop until the specimen is 



