it is this which is excited to further development by the appearance of the pollen-tube, and produces the embryo. Hofmeister had observed the organisation of the ovule, the nature of the embryo-sac and of the pollen-grain, and the formation of the embryo from the fertilised egg-cell step by step and cell by cell, and his account of these processes was aided by the light which Nägeli’s theory of the cell, and his reference of all processes of development to the processes of cell-formation, had thrown upon the history of development. He went on to apply the same method to the study of the embryology of the Muscineae and the Vascular Cryptogams, and followed the development of the sexual organs cell by cell in a large number of species; he observed the origination of the egg-cell which was to be subsequently fertilised, and the formation of spermatozoids, and above all he showed the divisions which take place in the fertilised egg-cell, and the relation of its segments to the further growth of the sexual product in course of formation. The whole course of development in the Muscineae and Vascular Cryptogams displayed a return twice repeated to the single cell as the starting-point in each case of a new phase of development; the true relation between the asexually produced spore and its germ-product on the one side, and the sexually generated embryo on the other, and their significance in the history of development, were brought out clearly by Hofmeister’s investigation, while the exactness of his method rendered lengthy discussions on the subject unnecessary. With these embryological processes, especially those of the Rhizocarps and Selaginellae, in which the presence of two kinds of spores was now for the first time correctly interpreted, Hofmeister compared the embryology of the Conifers, and by their aid that of the Angiosperms also.