Centrechinoida 555 



some hours prior to fertilization develop like freshly obtained ones.* 



Externally the sexes are indistinguishable. Just (1928) describes 

 several methods of obtaining gametes: ( 1 ) securing eggs shed normally; 

 (2) slightly injuring the animal, thereby stimulating it to shed "dry" 

 gametes; and (3) removing the gonads. The latter method is most 

 commonly employed. Just's method is to cut through the test slightly 

 above the equator, carefully removing the intact ovaries to seawater, 

 and allowing the eggs to shed without stimulating the gonads. The egg 

 suspension is then strained through cheesecloth. This is the procedure 

 also used by Dr. Chambers and Dr. Ethel B. Harvey. If, however, it is 

 necessary to obtain eggs in quantity from many individuals, the writer 

 has found that they may be obtained by the following method, which does 

 not impair their capacity for normal development in any way. The 

 animal is rinsed in tap water in order to kill any sperm which might be 

 on its surface, and then rinsed again with seawater ; a cut is made about 

 the peristome and the Aristotle's lantern is removed; the perivisceral 

 fluid is poured off and replaced with seawater. The ovaries of the fe- 

 males, which are distinguished by their deep red color, are broken loose 

 with a blunt instrument and poured into 250 cc. finger bowls of sea- 

 water, and are further broken up by drawing them gently in and out of 

 a wide-mouthed pipette. The eggs are washed several times prior to the 

 experiment. 



In a few minutes, after the eggs have settled, the writer pours off 

 the water and adds fresh water, repeating the process several times; 

 the last time, before the eggs have settled, they are poured into another 

 bowl, leaving any debris behind. Some workers state that ordinary 

 physical disturbances are harmful to the eggs, but the writer has found 

 no evidence of this. 



The males are distinguished by their yellowish testes. Each male is 

 placed, aboral side down, upon a dry Syracuse watch glass, into which 

 it soon exudes its seminal fluid. Dr. Ethel B. Harvey removes the testes 

 of several males to a small dish, which is kept covered to prevent their 

 drying. 



Eggs are fully maturated when shed. The polar bodies are usually 

 absent. The location of the animal pole is indicated by the markedly 

 eccentric position of the female pronucleus. The eggs are about 74M 

 in diameter. The jelly is 28 to 32^ thick, and may be demonstrated 

 by placing the eggs in a suspension of India ink in seawater. The 

 periviteline space is 3 to 5/* (Harvey, 1932). The presence of reddish 



♦Any one working with Arbacia eggs should consult Just's article (1928) concerning 

 methods of handling them, and also E. Newton Harvey's summary of 125 investigations 

 describing chemical and physical phenomena in the material (1932). The writer is 

 also indebted to Dr. Robert Chambers and Dr. Ethel B. Harvey for additional suggestions. 



