Among his especially noteworthy discoveries may be mentioned that of the egg of mammals (1827), and the notochord as occurring in all vertebrate animals. His discovery of the mammalian egg had been preceded by Purkinje's observations upon the germinative spot in the bird's egg (1825). Von Baer's Rank.--Von Baer has come to be dignified with the title of the "father of modern embryology." No man could have done more in his period, and it is owing to his superb intellect, and to his talents as an observer, that he accomplished what he did. As Minot says: "He worked out, almost as fully as was possible at this time, the genesis of all the principal organs from the germ-layers, instinctively getting at the truth as only a great genius could have done." [Illustration: Fig. 67.--Sketches from Von Baer's Embryological Treatise (1828).] After his masterly work, the science of embryology could never return to its former level; he had given it a new direction, and through his influence a period of great activity was introduced. The Period from Von Baer to Balfour In the period between Von Baer and Balfour there were great general advances in the knowledge of organic structure that brought the whole process of development into a new light. Among the most important advances are to be enumerated the announcement of the cell-theory, the discovery of protoplasm, the beginning of the recognition of germinal continuity, and the establishment of the doctrine of organic evolution. The Cell-Theory.--The generalization that the tissues of all animals and plants are structurally composed of similar units, called cells, was given to the world through the combined labors of Schleiden and Schwann. The history of this doctrine, together with an account of its being remodeled into the protoplasm doctrine, is given in Chapter XII. The broad-reaching effects of the cell-theory may be easily imagined, since it united all animals on the broad place of likeness in microscopic structure. Now for the first time the tissues of the body were analyzed into their units; now for the first time was comprehended the nature of the germ-layers of Von Baer. Among the first questions to emerge in the light of the new researches were concerning the origin of cells in the organs, the tissues, and the germ-layers. The road to the investigation of these questions was already opened, and it was followed, step by step, until the egg and the sperm came to be recognized as modified cells. This position was reached, for the egg, about 1861, when Gegenbaur showed that the eggs of all vertebrate animals, regardless of size and condition, are in reality single cells. The sperm was put in the same category about 1865.