108 THE MAMMALIAN EGG 



attributable to their purine and. pyrimidine bases. For work with 

 ultra-violet microscopy, it is necessary to have a powerful source of 

 radiation and an optical system composed of quartz. It is possible 

 to incorporate the phase-contrast principle in ultra-violet micro- 

 scopy, thus obtaining extremely good resolution of details in living 

 cells (Taylor, 1950; Smiles and Dobson, 1955). A difficulty inherent 

 in ultra-violet microscopy is that critical focusing cannot be done 

 by eye, except with expensive electronic scanning and cathode-ray 

 equipment, and so the common practice is to take a succession of 

 photographs passing through the estimated focal plane of the 

 selected detail. 



Eggs may be prepared for histological study in situ by placing the 

 ovary or Fallopian tube, or parts thereof, in a selected fixative, and 

 dehydrating and embedding in the usual way. This general pro- 

 cedure is the classical one, followed by Sobotta (1895), Van der 

 Stricht (1902), Rubaschkin (1905) and many others since. It is con- 

 venient and provides good permanent records, but it has disadvan- 

 tages : the state of the eggs cannot be examined before fixation, the 

 plane of the sections relative to internal structures of the egg is 

 entirely fortuitous, and it is often necessary to prepare rather a large 

 number of sections to be sure of including all the eggs in the 

 specimen. 



These disadvantages are overcome in the following ways : (a) The 

 eggs are recovered in the fresh state, by the means described earlier, 

 and examined and photographed under the low powers of the 

 microscope. They are then transferred to fixative solution in a 

 cavity-block. If required for electron microscopy, they are fixed 

 in a buffered solution of osmium tetroxide, passed through a series 

 of alcohol solutions, and finally into the monomer mixture, all 

 solutions being contained in cavity-blocks. Finally, the eggs are 

 deposited in some partially polymerized monomer mixture in the 

 lower half of a gelatin capsule (No. 00), and moved with a fine wire 

 into a close group at the bottom. The capsule is then placed in an 

 oven at 6o°C until polymerization is complete (1 or 2 days). Eggs 

 required for conventional microscopy are more easily handled by 

 a method such as that described by Dalcq (195 1). In a small Petri 

 dish, a mound of agar is built up; a cavity is produced in the top by 

 blowing a small bubble with a pipette while the agar is still fluid 

 and opening this later with a hot needle. The agar is covered with 

 fixative solution (Dalcq recommends alcohol : formalin : acetic acid, 



