The antheridia (Fig. 66. _J_, _M_) arise as simple hemispherical cells, in which two walls are formed (_K_ I, II), the lower funnel-shaped, the upper hemispherical and meeting the lower one so as to enclose a central cell (shaded in the figure), from which the sperm cells arise. Finally, a ring-shaped wall (_L_ iii) is formed, cutting off a sort of cap cell, so that the antheridium at this stage consists of a central cell, surrounded by three other cells, the two lower ring-shaped, the upper disc-shaped. The central cell, which contains dense, glistening protoplasm, is destitute of chlorophyll, but the outer cells have a few small chloroplasts. The former divides repeatedly, until a mass of about thirty-two sperm cells is formed, each giving rise to a large spirally-coiled spermatozoid. When ripe, the mass of sperm cells crowds so upon the outer cells as to render them almost invisible, and as they ripen they separate by a partial dissolving of the division walls. When brought into water, the outer cells of the antheridium swell strongly, and the matter derived from the dissolved walls of the sperm cells also absorbs water, so that finally the pressure becomes so great that the wall of the antheridium breaks, and the sperm cells are forced out by the swelling up of the wall cells (_N_, _O_). After lying a few moments in the water, the wall of each sperm cell becomes completely dissolved, and the spermatozoids are released, and swim rapidly away with a twisting movement. They may be killed with a little iodine, when each is seen to be a somewhat flattened band, coiled several times. At the forward end, the coils are smaller, and there are numerous very long and delicate cilia. At the hinder end may generally be seen a delicate sac (_P_, _v_), containing a few small granules, some of which usually show the reaction of starch, turning blue when iodine is applied. In studying the development of the antheridia, it is only necessary to mount the plants in water and examine them directly; but the study of the archegonia requires careful longitudinal sections of the prothallium. To make these, the prothallium should be placed between small pieces of pith, and the razor must be very sharp. It may be necessary to use a little potash to make the sections transparent enough to see the structure, but this must be used cautiously on account of the great delicacy of the tissues.