ON EGGS OF MARINE ANIMALS 45 



membranes is that with butyric acid as first used by Loeb on the 

 California sea-urchin. 



Which of these methods the worker will use depends upon the 

 objective of his experiment. Certainly, he will choose the most 

 reliable of them. In my experience, the methods employed by 

 the Hertwigs and Herbst are the least valuable. The three 

 remaining have value depending upon the purpose of the experi- 

 ment. If the aim is to obtain membrane-separation with the 

 least possible degree of induction of subsequent development of 

 the eggs, distilled water, butyric acid or other straight chained 

 fatty acids should be used because with these the eggs never 

 cleave; they only reach the monaster stage. The hypertonic 

 sea-water of strength sufficient to induce membrane-separation 

 will, if allowed to act long enough, induce not only membrane- 

 separation, but cleavage and subsequent farther development, 

 closely simulating the normal. 



In either distilled water or hypertonic sea-water the eggs 

 separate their membranes. With butyric acid-sea-water, on the 

 "other hand, membrane-separation takes place only after removal 

 of the eggs to normal sea-water. This difference in action of the 

 means the worker must bear in mind in order properly to interpret 

 whatever results he obtains on the basis of such experimentally 

 induced membrane-separation. 



To bring about membrane-sep arati on by means of distilled 

 water my procedure is as follows: 



Select eggs from a single female which by trial insemination 

 give perfect membrane-separation. Take a thick drop of eggs 

 in a Syracuse watch-glass mounted under low power of the 

 microscope and flood with distilled water. (It is best to use 

 distilled water which the worker himself has re-distilled in a 

 quartz-still; if such quartz distilled water is not available, use 

 tap water.) Now with the stop watch note the time when the 

 membranes are fully separated from the eggs. This observation 

 should be repeated several times. One learns that the time to 

 complete membrane-separation will vary depending upon the 

 species of echinid. Eggs of Arbacia in optimum physiological 

 condition separate membranes in distilled water in about 

 15 seconds. The moment that the membranes are fully off the 

 eggs, they should be brought into a large volume of sea-water. 



