786 Transactions of the Society. 



It was not, however, a success, and for some time I did not 

 attempt the experiment of preserving marine ova. But three 

 or four years later I was brought into constant contact with 

 quantities of marine material, and as I overhauled the so-called 

 " refuse " of scallop-boats for the purpose of selecting the Crustacea 

 already referred to, I regretted that some means could not be found 

 for preserving the many beautiful objects that I was obhged to 

 throw away. 



Besides this there were so many points of interest connected 

 with the Crustacea themselves that required microscopic investiga- 

 tion, that it was absolutely necessary to do something to try and 

 retain them. 



Without attempting to go into details, apart from the im- 

 mediate subject of my paper, I should like here to mention that as 

 regards the Crustacea alone, not only is it possible to identify a 

 species by an examination of its ova, but even the habits and mode 

 of life can be in most cases determined by such examination. 



As regards fish embryos, the importance of being able to 

 preserve them easily for the Microscope cannot be overestimated, 

 whilst for less important forms the bare fact that the zooea of 

 Crustacea were once thought to form a separate order of animals, 

 shows how necessary a knowledge of such comparatively unfamiliar 

 objects is. 



With the desire therefore to retain these ova for microscopical 

 examination, I tried many methods, all of which resulted more or 

 less in failure ; the means I now adopt, though perhaps not perfect, 

 is at any rate suitable for the purpose, chiefly I believe owing to 

 the strength of the cement employed. This cement is composed of 

 2 parts carbonate of lead or white lead, 2 parts red oxide of lead 

 or minium, and 3 parts of litharge or oxide of lead. These parts 

 should be ground very finely, mixed dry, and kept so in a wide- 

 mouth bottle ; when required for use, a little of this powder should 

 be mixed in a small china pan with gold size to the consistency of 

 ordinary paint : care being taken that no trace of grit or unground 

 matter exists, for the slightest atom of this will prevent the cover- 

 glass from settling down firmly to the cell, and cause an influx of 

 air at once. 



The first operation is to fix a cell to a glass slip by means of 

 this cement ; as soon as it is set apply a coat of the cement to the 

 upper surface of the cell and to the outside angle of the same ; 

 this done, the slide should be put aside in a dry place to harden, 

 and I consider it best to allow this to occupy a fortnight at least 

 before further use, by which time there will be no fear of any 

 leakage or fault ; in fact, the cell will have become so firmly fixed 

 that it can be entirely filed off", if a metal one, without breaking it 

 away from the cement. 



