thin collodion solution and add thick (6%) solution. In this there is 

 gradual concentration of the solution in the tissue. Allow small 

 specimens to remain a day, or, better, several days ; 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 

 specimens, 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 until the solution has at- 

 tained the right consistency. To accomplish this it is only neces- 

 sary 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 solution has itself thickened by evaporation, with large 

 specimens it is advisable to follow the 6% bath with a stay in a 

 thicker solution, as 8%, for a day or so. 



§ 55. Imbedding. Pour off the 6% solution and add for a 

 short time at least an 8% solution of collodion. The tissue is now 

 ready for imbedding in 8%, which may be accomplished in either of 

 two ways : (a) on a cork or other holder that may be clamped in the 

 microtome, 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 (cork). Choose a cork of a convenient size ; 

 put a drop or two of collodion upon one end and insert a pin verti- 

 cally to the surface near the edge. Transfer the tissue from the 

 vial of thick collodion to the cork and lean it against the pin. The 

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

 thick collodion onto the tissue, drop by drop, moving the cork in 

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

 and envelop the tissue. Continue to add drops of collodion at in- 

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

 slight film hardens on the surface invert the cork bearing the tissue 

 in a shell-vial of large diameter containing enough chloroform to 

 float the specimen and cork. If the piece of tissue is of awkward 



