ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



461 



brought to the beginning, or to any intermediate position, and then 

 held. The knife, which is bilaterally symmetrical in section, has a 

 straight edge, and is 315 mm. long. 



Ott Rotary Microtome. — This instrument (fig. 91) was described 

 in the June Xumber (see page 30;!). 



Fig. 91. 



Gaskell's Method of obtaining Frozen Sections after embedding 

 in Gelatin.* — The tissue is fixed in any formalin mixture, such as 10 p.c. 

 formalin solution in normal saliue, or formol Miiller solution. The tissue, 

 when fixed, is thoroughly washed out in running water for 12 to 24 

 hours, to remove all formalin. It is then embedded at 37° C. in an 

 ordinary incubator, in gelatin previously soaked for 3 to 4 minutes in 

 cold water, and then melted. The embedding process continues for 

 2 to 5 hours — longer periods are unnecessary and harmful. The tissue 

 is then cast in paper boxes in the gelatin in which it has been soaked. 

 The mass is then allowed to set at room temperature, and then transferred 

 to a formalin vapour chamber to harden. It is left in this for a period 

 of at least 3 days ; it may, however, remain in the chamber almost in- 

 definitely, and this is a convenient way of keeping the blocks till wanted. 



To obtain sections, the paper is cut away with a knife, and the block 

 pared down, and then attached to the stage of Aschoff's freezing micro- 

 tome, as made by Sartorius, by a drop of gum solution, and frozen. 

 Sections can be obtained of any tissue hitherto tried of a thickness of 

 10 fj., and of most tissues o-/>i sections are obtainable. They are floated 

 on cold water, and can then be stained in any manner required. The 

 staining of the gelatin is not found to cause much inconvenience. The 



* Journ. Path, and Bacterid., July 1912. 



