SECTIONING AT A TEMPERATURE OF 25 FAHRENHEIT. 285 



ing only a few days' work to prepare for study as needed. We 

 always use very small blocks with four to six eggs arranged in a 

 row in each block, and never more than one pair of ovaries in 

 one block, as we find difficulties multiply with the increased size 

 of the blocks, sections from the smaller blocks showing less ten- 

 dency to curl or break. 



It might seem that placing the microtome near an open window 

 on a cold day would accomplish all we claim for the freezer, but 

 the draught from an open window is sure to break the parafifine 

 ribbon. Even in a cold room with closed windows, it is quite 

 impossible to exclude currents of air, and the warm breath of the 

 operator close to the microtome is a dangerous factor. In the 

 freezer, the operating compartment is so nearly air tight that air 

 currents are practically excluded. When the weather is intensely 

 cold, we have sometimes used the freezer without ice, merely as 

 a cover for the microtome, to enable us to section close to an 

 open window, but we have always obtained the best results by 

 using ice, and an even temperature as near 25 F. as possible. 



For several years we have used this method of cutting thin 

 sections of the egg of Alloloboplwra, for we had found it impos- 

 sible to secure satisfactory thin sections of this egg with the 

 methods in common use. Other investigators, however, rarely 

 complain of difficulty in obtaining thin sections, and this may be 

 due to fixing and hardening their material to a degree that makes 

 any further hardening of the imbedded mass unnecessary, or to 

 the fact that a large number of cells imbedded in one block of 

 itself presents a harder mass than the single row of eggs we 

 imbed in each block. 



In our material these thin sections have been of the greatest 

 value in aiding the interpretation of several obscure points ; the 

 constancy of the centrosome, for instance, was not demonstrated 

 for this egg until we had secured sections of 3 u. or less. And 

 for photographic reproduction at high magnification, thin sections 

 offer very decided advantages, 3 fjt representing the maximum 

 thickness we have been able to use most successfully. It is pos- 

 sible of course to photograph the different planes of a thick 1 as 



1 With our method of focussing, a thick section requires a focussing (minus spher- 

 ical) lens of lower power than the one used for thin sections. 



