Casting blocks 



NITROCELLULOSE SECTIONS 



145 



Fig. 82. Transferring object in thick celloidin. 



celloidin; that is, if the object were to take 

 one hour in 2 % it should have eight hours 

 in 16%. As a measure of absolute time it 

 may be said that an object the size of a 

 frog's egg will require about one day in the 

 2 % while a flower bud the size of a walnut 

 should remain at least ten days. The 

 writer prefers not to endeavor to transfer 

 the object from one solution to another, 

 but to pour off the weak solution and re- 

 place it ^\'ith a stronger. There is no means 

 of telling when impregnation is complete 

 until one comes to cut the section; but 

 two things must be remembered: one, that 

 the specimen cannot be damaged no mat- 

 ter how long it be immersed in celloidin, 

 two, that the most frequent cause of faulty 

 sections is imperfect impregnation. When 

 impregnation is complete it is necessary 

 to prepare a block for cutting. 



Casting Celloidin Blocks 



Two methods may be employed to 

 transform the celloidin from a hquid to a 

 solid state. Either the alcohol-ether mix- 

 ture may be permitted to evaporate, or it 

 may be removed with another solvent, 

 usually chloroform, in which celloidin is 

 not itself soluble. The removal of the sol- 

 vent with chloroform may also be done 



either in the hquid or in the vapor 

 phase. 



It is difficult to mount blocks of celloi- 

 din once they have been cast on an object 

 holder, though the solution of Apathy 

 (Chapter 27, E 21.1 Apathy (1942)) has 

 been specifically developed for that pur- 

 pose. It is, moreover, very nearly im- 

 possible to mount a celloidin block on 

 any of the metal block holders supplied 

 with standard microtomes. The worker is, 

 tlierefore, advised to prepare for himself 

 a series of small wooden blocks on which 

 the object may be directly cast. These 

 wooden blocks should be of hardwood and 

 a whole series should be provided accord- 

 ing to the size of the object which is to be 

 cut. The smallest practical size is about 

 J2" X H" X 1" and it should be under- 

 stood that the block will be cast on the 

 J-^-inch end. The largest practical size is 

 dependent entirely upon the size of the 

 object to be cut. These blocks should be 

 cut, sanded as smooth as possible, baked 

 in an oven at about 80°C. to remove as 

 much water as possible, and then thrown 

 directly into absolute alcohol. The abso- 

 lute alcohol is replaced with absolute alco- 

 hol-ether and then with 2% celloidin. The 

 blocks are transferred for se\'eral daj's to a 

 solution of 4% celloidin and then with- 



