4 BASIC METHODS FOR EXPERIMENTS 



of the size of those of Arbacia or Cumingia, it is preferable to 

 use stenders of lOO cc. capacity or less, depending upon the 

 number of eggs. 



In selecting the dishes one should also consider the species 

 of egg under investigation. Some eggs develop normally or 

 not depending upon the volume of sea-water containing them. 

 Thus the eggs of Asterias, removed from the ovary, go through 

 most normal maturation when placed in larger volumes of sea- 

 water. (Just, 1929.) For the eggs from one ovary of Asterias 

 I used flat-bottom dishes which easily hold 3,000 cc. of sea-water. 

 On the other hand, eggs of Platynereis megalops will fertilize 

 only when in a volume of sea-water equal to and not surpassing 

 the volume of eggs. (Just, 191 5.) 



Utilization of Sea-Water 



Precautions must be taken to insure as far as possible uni- 

 formity with respect to the temperature and gas content of the 

 sea-water. Heilbrunn's practice of drawing off in a large flask 

 the sea-water that he is later to use in an experiment is excellent. 

 In the first place, this sea-water soon comes to room temperature 

 or at least insures that all dishes in the series of a given experi- 

 ment have sea-water of the same temperature. Secondly, sea- 

 water is frequently charged with gas as it comes from the taps. 

 Therefore, if we follow Heilbrunn's method, by the time that 

 we are ready to use the sea-water, the gas bubbles have dis- 

 appeared. When present they are a nuisance and may be even 

 harmful. Sea-zvater should never be drawn from the tap directly 

 on to the egg suspension; the eggs may thus be injured. 



Dishes containing eggs should always be protected against 

 evaporation, because this makes the sea-water hypertonic and 

 hypertonic sea-water is itself an experimental method. (Just, 

 1928.) Therefore the dishes should be covered. On warm days 

 it is well to keep such dishes on the live table in running sea- 

 water which, however, should never be too deep. Where eggs 

 are in cylinders, test tubes or small dishes, they may be placed 

 in a larger dish with wetted sea-sand on to which a gentle stream 

 of sea-water flows. The containers, of course, must be covered 

 to avoid chance entrance of sea-water, and precautions must be 

 taken against overflow. That is, the cylinders, tubes or other 



