Ten years after Baer had given a firm foundation to embryological science by his theory of germ layers a new task confronted it on the establishment of the cellular theory in 1838. What is the relation of the ovum and the layers which arise from it to the tissues and cells which compose the fully developed organism? The correct answer to this difficult question was given about the middle of this century by two distinguished pupils of Johannes Müller--Robert Remak, of Berlin, and Albert Kölliker, of Würzburg. They showed that the ovum is at first one simple cell, and that the many germinal globules, or granules, which arise from it by repeated segmentation, are also simple cells. From this mulberry-like group of cells are constructed first the germinal layers, and subsequently by differentiation, or division of labor, all the different organs. Kölliker has the further merit of showing that the seminal fluid of male animals is also a mass of microscopic cells. The active pin-shaped "seed-animalcules," or _spermatozoa_, in it are merely ciliated cells, as I first proved in the case of the seed-filaments of the sponge in 1866. Thus it was proved that both the materials of generation, the male sperm and the female ova, fell in with the cellular theory. That was a discovery of which the great philosophic significance was not appreciated until a much later date, on a close study of the phenomena of conception in 1875. All the older studies in embryonic development concern man and the higher vertebrates, especially the embryonic bird, since hens' eggs are the largest and most convenient objects for investigation, and are plentiful enough to facilitate experiment; we can hatch them in the incubator, as well as by the natural function of the hen, and so observe from hour to hour, during the space of three weeks, the whole series of formations, from the simple germ cell to the complete organism. Even Baer had only been able to gather from such observations the fact that the different classes of vertebrates agreed in the characteristic form of the germ layers and the growth of particular organs. In the innumerable classes of invertebrates, on the other hand--that is, in the great majority of animals--the embryonic development seemed to run quite a different course, and most of them seemed to be altogether without true germinal layers. It was not until about the middle of the century that such layers were found in some of the invertebrates. Huxley, for instance, found them in the medusæ in 1849, and Kölliker in the cephalopods in 1844. Particularly important was the discovery of Kowalewsky (1886) that the lowest vertebrate--the lancelot, or amphioxus--is developed in just the same manner (and a very original fashion it is) as an invertebrate, apparently quite remote, tunicate, the sea-squirt, or ascidian. Even in some of the worms, the radiata and the articulata, a similar formation of the germinal layers was pointed out by the same observer. I myself was