LOCATION OF LUMINOUS MATERIAL When a noctiluca is giving a bright, constant glow—for instance, when treated with n/2000 to n/4000 HCI—it is fairly easy to observe the light under a microscope in a dark room. The chief luminescence comes from the main mass of protoplasm near the mouth-groove and around the periphery of the cell, while the area between these two regions is much fainter. The luminescence is a general glow over the cell, and a similar glow is suffused momentarily over the cell when a noctiluca is stimulated by a needle. This effect is, however, probably due to a close aggregation of small luminous particles, for when noc tilucas are crushed under the microscope and the particles separated by means of a cover-slip, numerous points of light may be observed, similar to stars in the sky. The luminescence, therefore, apparently comes from small granules in the protoplasm, which may be freed from the cell by crushing. That the luminous granules are located in the protoplasm and not elsewhere was shown by stimulating cells injured by electricity. As mentioned previously, a few shocks of an induced current causes the protoplasm of a noctiluca to shrink away from the membrane into a compact irregular mass; when again stimulated, the irregular mass of protoplasm gives light, while the clear peripheral area remains entirely dark. Also, in centrifuged noctilucas there is
a gradual motion of luminescence in a cell corresponding roughly with the motion of the protoplasm as it resumes its normal position. These granules in the protoplasm to which the luminescence may be traced do not stain with neutral red nor with methylene blue; there are other granules, however, especially around the periphery of the cell, which do stain with neutral red.
It has not been possible to obtain two substances concerned with light-production in Noctiluca, as in Photos (Dubois, 1913, 1914), firefly and Cypridina (E. N. Harvey, 1917), etc., one destroyed by heat (pho togenin) and the other thermostable (photophelein), notwithstanding repeated efforts to demonstrate them. Only one substance respon sible for light-production in Noctiluca can be demonstrated, and this substance occurs as granules and burns until it is all used up as soon as it is brought into contact with atmospheric oxygen by crushing the cells; this we may conveniently call photogenin.