Abstract
For several years experiments have been made with the germ cells of the tropical sea urchins Toxopneustes and Hippanoe, and the northern urchin Arbacia. The preliminary experiments were made with a view towards obtaining experimental conditions that were optimum and that gave the least variability for freshly removed eggs and sperm from freshly collected sea urchins. Surprisingly large differences were observed among the eggs from different females. These differences involved (1) the size of the eggs, (2) the presence of the jelly layer, (3) rate of membrane formation, and (4) cleavage. By means of one or more of these criteria it was possible to grade the different freshly collected females according to the physiological condition of their eggs. Eggs of similar physiologic condition showed a minimum variability and the highest correlation with respect to these variants. When eggs and sperm were removed from their respective bodies and kept under optimum laboratory conditions, the same changes that had begun within the bodies of the sea urchins continued outside of the body. With increasing age outside of the body the eggs showed progressive changes in size, in loss of jelly, in retarded membrane formation, in decreased total cleavage and decreased rate of cleavage, etc. And the exact degree of change in these regards was ascertained for different intervals up to the death of the eggs. Still other changes were consequent upon ageing of the eggs, which suggested the nature of the chemico-physical agencies involved in the ageing process: namely, agglutination, fusion of two or more eggs, separation of the blastomeres, and irregular cleavage. These changes suggested that the excess free HO ions in the sea water was one agency and probably a very important one in causing the dissolution of the jelly, the changes in permeability of the cortical layer of the eggs, the changes in size, and all the other changes mentioned above, that follow upon long exposure to the free HO ions.
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