Nature of Hyaline Membrane in Fertilized Egg of Sea Urchin.

The inner or hyaline membrane of the fertilized egg of the sea urchin, Strongylocentrotus purpuratus, which serves to hold the blastomeres together, is rendered permanent by keeping the developing eggs in a solution composed of 35 cc. sea water + 15 cc. 3/8 M Ca Cl2. Such cultures show the following facts: (1) The outer or fertilization membrane around the inert blastula disappears normally, i. e., within 24 hrs. It is therefore not broken by the hatching blastula, as is generally supposed. (2) So long as the larvae remain within the Ca-membrane they retain the form of blastulae, but segmentation goes on, and eventually the blastocoel is filled with mesenchyme cells; invagination does not occur. (3) After 3 to 5 days the larvae burst the membranes by pressure; the point of breaking has no reference to the axis of the animal. (4) After escape, the larvae invaginate and form more or less abnormal plutei; the insoluble membranes lie about on the bottom of the dish. The fertilized eggs of the sand urchin, Dendraster eccentricus, since they have no hyaline membrane, do not show similar reactions. In the calcium sea water solution they develop normal gastrulae and plutei only a little more slowly than the controls. In this sand urchin, therefore, the blastomeres must be held together by a mechanism different from that of the sea urchin. If the fertilized eggs of sea urchin are put into sea water which has been brought to pH 3.5 by means of HCl, the hyaline membrane disappears, and in the course of a few minutes the perivitelline space becomes filled with hyaline droplets exuding from the cytoplasm. In volume the droplets may come to equal three-fourths the volume of the cytoplasm, but neither the diameter of the tgg (.072 mm.) nor of the sphere enclosed by the outer membrane (.098 mm.) is immediately altered. Therefore the droplets must largely consist of the sea water of the perivitelline space transferred into the droplets, presumably by osmosis.