PRACTICAL DIRECTIONS, [APP. 



pieces of lead, placed on a glass slide in such a 

 manner as to enclose a space, The latter is 

 preferable because the object can be placed 

 in any position required with great ease by 

 moving it with a hot needle, and the whole can 

 be cooled rapidly. It is advisable, at any rate at 

 first, to arrange the embryo so as to cut it into 

 transverse sections. 



When cool a block of paraffin is formed, in 

 the midst of which is the embryo. 



Other imbedding agents have been used. The best 

 of these are, (1) pure cocoa butter ; (2) a mixture of 

 spermaceti and castor oil or cocoa butter (4 parts of 

 the former to one of the latter). With these imbedding 

 substances, it is generally necessary to moisten the razor, 

 either with olive oil or turpentine and ribbons of sec- 

 tions cannot be made (see b). 



Cutting sections. 



When the imbedding block is cold pare away 

 the edges, then gradually slice it away until the 

 end of the embryo is near the surface, and 

 place it in a microtome. 



The microtome we are most accustomed to is 

 a ' sliding microtome' made by Jung of Heidel- 

 berg ; it gives excellent results. Recently how- 

 ever Messrs CaldweH and Threlfall have designed 

 an automatic microtome which has been used 

 with success at the Cambridge Morphological 

 Laboratory and promises to effect a great saving 

 of time and trouble in cutting sections (vide p. 471 

 and Proceedings of the Cambridge Phil. Soc. 1883). 

 A convenient small microtome is one made by 

 Zeiss of Jena (also by the Cambridge Scientific 

 Instrument Company), in which the object is 

 fixed and by means of a finely divided screw 



