^ 



126 Papers from the Marine Biological Laboratory at Tortugas. 



The first of these methods was tried at the Tortugas laboratory on the 

 eggs of the sea-urchin Lytechinus {Toxopneustes) variegatus. Advantage 

 was taken of the fact that the electrolytes of the eggs and the medium 

 dissociate into ions which bear electric charges, and therefore their move- 

 ment through the eggs could be detected, with a high degree of accuracy, 

 by the electric conductivity method of Kohlrausch.^ 



Several bushels of sea-urchins were collected each day and the eggs of 

 the ripe females placed in large dishes of sea-water. The mucus egg 

 membranes (jelly, zona pellucida) were washed off and the eggs were intro- 

 duced into a conductivity vessel made especially for the purpose 

 (fig. i). The conductivity vessel was placed in the centrifuge 

 and the eggs were precipitated until they were closely packed 

 together. By microscopic examination it was found that only a 

 trace of sea-water was left in the spaces between the eggs (fig. 2). 

 By raising the electrodes above the level of the eggs they came 

 to lie in the supernatant sea-water that had been pressed out 

 from between the eggs. In this way the conductivity of sea- 

 water and egg-mass could be measured separately within two 

 minutes. The conductivity of the egg-mass, when moderately 

 centrifuged, was about one-twentieth of that of the sea-water, in- 

 dicating that the conductivity of the egg itself must be almost nil. 

 The conductivity of the egg-mass was greatly affected by the 

 degree of packing. However, the vessel was 

 so long and narrow that the packing of the 

 eggs could be recorded accurately by mark- 

 ing their upper limit on the side of the vessel 

 (fig. I, a). The eggs could then be removed 

 from the conductivity vessel, replaced, and 

 centrifuged to the same level, when the con- 

 ductivity was found to be the same. In this 

 Fig. I. Fig. 2. way I was able to measure the conductivity 



before and after fertilization and before and 

 after artificial stimulation causing cleavage. It was found that the conduc- 

 tivity increased about one-fourth when the developmental processes began. 

 The sources of error and methods of controlling them are discussed in another 

 paper. 2 



One point, however, the elimination of the fertilization membrane, needs 

 further consideration. Soon after the egg is fertilized, a fine line appears 

 close around it and gradually expands until it forms a circle, widely separated 

 from the egg. Almost every one (except Kite) supposes this to represent 

 a spherical membrane, the "fertilization membrane." Before fertilization 

 the egg is covered by a thick transparent jelly or mucus layer, called the 

 chorion or zona pellucida by some authors. I stained this jelly with basic 

 dyes (neutral red and methylene blue). These dyes coagulate the jelly, 

 causing it to become membranous. 



' Ostwald-Luther. Messungen. 'McClendon. Amer. Jour. Physiol., vol. 27, p. 240. 



