The following account of the oogenesis refers especially to Fucus vesiculosus, F. serratus , and Ascophyllum nodosum , except where the contrary is stated:— The first sign of the young oogonia consists in the projection of the cells forming the wall of the conceptacle into its cavity. The protoplasm of the papilla in well-fixed material appears finely granular, and in very thin sections it can be seen that this peculiarity is due to a reticulate or alveolar structure, the latter explanation being by far the most probable one. The nucleus is large, and is often excentrically situated in the cell. It is difficult to procure good material for the study of the division of this nucleus, but it is most important to be sure of the chief processes connected with its karyokinesis. It is best to collect the plants shortly after they have been covered by the flowing tide; indeed, we found it to be almost impossible to secure the required stages at any other time. Our own practice has been to take a boat, and to gather and fix the material on the spot, cutting up the receptacles of the fertile thalli, and dropping the pieces at once into the fixing solution. As the result of a lengthy series of trials we found the best material was obtained by using either Flemming’s (strong) or Hermann’s solutions with half the normal amount of osmic acid. We obtained very good results by making up these fixatives in 50 per cent. sea-water. When the projecting papillæ are about to enter on their first stage of division the nucleolus is seen to become vacuolated, although it does not disappear till just before the grouping of the chromosomes in the equatorial plane, at which stage we could no longer distinguish it. But the fact must not be lost sight of that it loses part of its contents at a very early period of the karyokinetic process.