FERTILIZATION REACTION IX ECHINODERM EGGS. 255 



fertilizable as they exude from the genital pores, and yield 100 

 per cent, fertilization membranes, cleavage, and larvae. This 

 agrees with observations on other starfish. As is well known, 

 and this is also the case in Patiria, starfish eggs obtained by shak- 

 ing or mincing the ovaries are not mature, but must stand in 

 sea-water for some time before they attain a fertilizable condition. 



In the following account a number of statements are made 

 concerning viscosity differences. These have been determined in 

 the following manner. The eggs are placed on a slide in a drop 

 of sea-water without cover. Low power was used (Leitz ob- 

 jective 2, ocular 3). Any egg to be investigated was rapidly 

 pushed with a needle to the periphery of the drop and then into 

 a small evagination of the periphery where it is held by surface 

 tension. It was then punctured or cut with a needle and the rate 

 and readiness with which the egg cytoplasm flows out under the 

 pressure of surface tension as well as the duration of retention 

 of cuts and gashes furnish a relative measure of the viscosity of 

 the cytoplasm. The needle used w r as a fine steel needle thrust 

 into a wooden handle and operated by hand. The method is 

 somewhat crude but has the advantages that it is rapid and almost 

 entirely objective. 



2. Viscosity of the Unfertilized Mature Egg. The unfertilized 

 egg in all three species consists of a slightly viscous cytoplasm 

 inclosed in a definite membrane of considerably greater consist- 

 ency than the cytoplasm. This membrane, which may be de- 

 signated the vitelline membrane (it has also been named plasma 

 membrane and egg membrane) is probably a colloidal gel. It is 

 more delicate in the sea-urchin than in the starfish egg. 



These facts have been determined as follows. When the sea- 

 urchin egg (either species) is held by surface tension and punc- 

 tured with a needle, the cytoplasm rushes out with almost ex- 

 plosive force and the whole egg very rapidly disintegrates. In 

 this disintegration the vitelline membrane is also involved so 

 that I at first thought such a membrane was absent. However, 

 if the eggs are rapidly pushed back into the drop when they are 

 partially disintegrated, egg fragments of various sizes are ob- 

 tained. On the surface of such fragments wrinkles are always 

 observable. It is evident that such wrinkles must be located in 

 a surface membrane of greater consistency than the cytoplasm, 



