Page:Popular Science Monthly Volume 86.djvu/574

570 observed that the frog's egg may be made parthenogenetic by means of a momentary electric shock. Unfertilized eggs of a frog were divided into two equal lots, placed in distilled water, and one lot shocked electrically. It was found that three times as much salt diffused out of the shocked eggs as out of the control. Since the salt must have come from the interior of the eggs, the experiment seems to prove that the eggs must have been permeable to it. The shocked eggs began to segment and behaved in other ways as if normally fertilized.

There seems to be no doubt that the permeability of the egg is increased by agents producing parthenogenesis, but just how this influences the egg's development is not absolutely settled, because of the many processes in development which are very far from being solved. Some of these processes have been the subject of numerous investigations. The outward change in form during the segmentation of the egg is caused by changes in surface tension. Granted that fertilization alters the permeability of the egg, it may be that the changes in permeability influence the surface tension. The unfertilized eggs of sea urchins, frogs and many other animals are surrounded by jelly-like coats, the inner layer of which lies close to the egg. On fertilization, the jelly is pushed out by the perivitelline fluid exuding from the egg, the space occupied by the fluid being called the perivitelline space. The inner layer of the jelly looks like a distinct membrane and is called the "fertilization membrane." Loeb considers its formation of great importance. Biataszewicz has shown that the frog's egg shrinks as this fluid is "secreted." Glaser has observed the same phenomenon in the sea urchin's egg, though the shrinkage is so slight that other observers deny its taking place. Granting that the perivitelline fluid comes from the egg, the increase in permeability would facilitate its migration.

If the jelly be removed from the sea urchin's egg prior to fertilization, no "fertilization membrane" appears. Presumably the fluid is secreted but lost in the surrounding water. Though the membrane helps to protect the embryo, its existence is not absolutely essential, since eggs lacking it (due to the removal of the jelly) have been known to develop. Many observations and experiments have demonstrated to the writer that the tough "fertilization membrane" of the sea-urchin's egg does not exist (at least in its final condition) before fertilization. The increase in permeability allows the escape of the perivitelline fluid which, according to the hypothesis advanced in 1911, interacts with the jelly and forms the "fertilization membrane." Elder, in 1913, came