The ripe fish, Fundulus heteroclitus, are found in abundance at Woods Hole from the middle of June until the first week in August. The fish were stripped, the eggs immediately fertilized and kept in sea water a t room temperature. About fifteen minutes after fertilization the germdisc becomes clearly visible although its limits are not sharply defined until from one hour and fifteen minutes to one hour and twenty-five minutes after fertilization. The first cleavage usually takes place about two hours after fertilization and the second cleavage plane appears about thirty minutes later. The rate a t which cleavage takes place seems to vary with the temperature of the room.
The operations were very simple and were done under the binocular microscope. The eggs were held by a small pair of forceps with sufficient pressure to prevent the yolk from turning about within the vitelline membrane. With a very fine needle a puncture was made through the vitelline membrane into the germdisc or blastodisc a t the point desired and as the needle was withdrawn a slight compression with the forceps would send out a stream of protoplasm from this region. The extent and position of the defect thus made in the egg can easily be seen through the vitelline membrane.
The changes following the operation, namely a rapid closing together of the tissue surrounding the defect, are so rapid that 1 These experiments were done a t the Marine Biological Laboratory, Woods Hole, Mass.