ON EGGS OF MARINE ANIMALS 5 



vessels should be placed in the sand in such wise that the tops 

 are well above the surface of the sand. 



Dishes containing eggs should, of course, be protected 

 against direct sunlight. 



Thermometers 



It is well to record both the room temperature and that of 

 the sea-water containing the eggs. One should therefore have 

 available two standardized thermometers. If possible, one 

 should use such which read in tenths of degrees. The room 

 temperature should be taken always in the same place in the 

 room which should be protected against sunshine. The dishes 

 containing the eggs, the temperature of which is taken, should 

 be similarly placed and protected. In view of the fact that 

 change in temperature is a very important experimental means, 

 the worker should keep careful record of the temperature at 

 which he conducts his experiments. 



Optical Equipment 



Many workers seem to think that because they are working 

 at the sea-side they should use the most disreputable and 

 obsolete microscopes and lenses. Nothing is farther from the 

 truth. If one is seriously engaged with the embryology of 

 marine animals one must study the living eggs. The best 

 optical equipment possible is therefore none too good. Accord- 

 ingly I use the finest apochromatic lenses and compensating 

 oculars procurable. A good darkfield condenser and a plankton 

 condenser are valuable accessories for the study of the living 

 sperm and eggs. For measuring, I use a screw micrometer 

 with compensating ocular. Of course, such susceptible instru- 

 ments need, particularly at the sea-side, special care. 



Water immersio?i lenses 



A good apochromatic water immersion lens is a valuable aid 

 for the study of living eggs. However, a word of caution must 

 be said: the lens coming in contact with sea-water must not be 

 mounted in copper or other metal because by such mounts the 

 sea-water is rendered toxic. Procure therefore a water immer- 

 sion lens in a special non-toxic mount. 



