ON EGGS OF MARINE ANIMALS 73 



for a well-fixed egg which is a mixture of components of varying 

 degrees of consistency one must have a real microtome knife 

 and not a makeshift, especially for thin sections. Secure there- 

 fore a heavy knife of the very best steel; both money and time 

 are thus eventually saved. I use knives of not less than 12 cm. 

 in length, 6 cm. in height; frequently for very brittle and hard 

 objects those of 26 by 8 cm. 



For cutting, the knife is most firmly clamped. The knife- 

 carriage is then brought to within two millimeters of the forward 

 edge of the paraffin block. With even, gentle strokes the wheel 

 is turned with the cam set at 15 to 20 microns until the first 

 section is cut. The cam is now reset for the thickness of the 

 sections desired and cutting begun anew. Since these first 

 slices contain no sections of eggs, needed adjustments can be 

 made — of the alignment of the block and the tilt of the knife. 



For cutting serial sections the knife should be slightly 

 tilted toward the paraffin block. It is difficult to set a definite 

 rule concerning the degree of tilt: so much depends upon the 

 size, weight and form of the knife as well as upon the hardness 

 of the paraffin and the heterogeneity of the object. One must 

 therefore by experience avoid excessive tilt. Set the knife at 

 that angle which gives a smooth ribbon in which the sections of 

 eggs are perfectly intact. And as always be sure that the edge 

 of the knife is as sharp as possible. Despite the utmost care 

 one often includes with marine eggs, especially those of echino- 

 derms and of tubiculous worms, particles of grit or sand which 

 in cutting nick the knife. For this reason, one must change 

 the knife to a new position the moment that a rift appears in 

 the paraffin ribbon; if this is not due to an air bubble or drop 

 of water it is probably due to some hard foreign particle. In 

 the long run it is safest and most economical to have experts 

 resharpen the knives. 



The sections should be cut without the loss of a single sec- 

 tion in one continuous ribbon whose length is sufficient for 

 mounting on one slide. The ribbon is removed with camel's 

 hair brushes on to smooth hard paper and cut into two, three 

 or four strips, depending upon the width of the ribbon, each of 

 which, allowing for the stretching of the paraffin, will make a 

 row of sections slightly shorter than the length of the cover slip 



