ON EGGS OF MARINE ANIMALS 71 



With the cooled alcohol the paraffin never cracks. But do not 

 allow the alcohol to become too cold. 



The paraffin block is now labelled by scratching with a fine 

 needle on each flat surface near the mass of eggs. It is then 

 trimmed with a sharp knife with that surface of the block upper- 

 most nearest which the eggs lie. Along one side the paraffin 

 is cut away to a tangent one millimeter away from the black 

 patch of eggs. From the edge opposite only enough paraffin is 

 removed to make this parallel to the first. On the other sides 

 the block is so trimmed that it assumes wedge-shape, the upper 

 borders of the inclined planes coming not closer than one milli- 

 meter to the eggs at that point where they lie closest to the 

 surface first cut. Thus one prepares a wedge-shaped block in 

 which the eggs show up nearer one parallel edge than the other; 

 these edges we may designate cutting edge and affixing edge 

 respectively. 



The face of the block opposite that nearer which the eggs 

 lie may now be trimmed with a very sharp knife. The trim- 

 ming, accomplished by paring away less than a millimeter of 

 the paraffin at a time, extends only about one third the distance 

 from the cutting edge to the affixing edge in order that this 

 latter may be solidly stuck to the mounting disc of the micro- 

 tome. The paraffin is pared away evenly so that the faces of 

 the block are as nearly parallel as possible, the block having a 

 thickness between the two faces of about two to two and one 

 half millimeters. Any further trimming is accomplished after 

 the block is affixed. 



The block is now fixed to the disc. This is accomplished by 

 dipping the surface of the disc into melting paraffin and gently 

 pressing the paraffin block on to this surface as the melted par- 

 affin congeals. Once the joint is made, it is further strengthened 

 by surrounding it with melted paraffin. The mass of paraffin 

 should now be one fused block. The disc is then put upright in 

 a cool place — but never in cold water for this breaks the joint! 

 — and allowed slowly to solidify. If the mount lies on the side 

 during this process of cooling the paraffin block may bend; for 

 this reason I prefer to use discs that can stand upright instead 

 of those the ends of whose shanks are convex. 



