Pictures of Cells in the Seventeenth Century.--The sketches illustrating the microscopic observations of Malpighi, Leeuwenhoek, and Grew show so many pictures of the cellular construction of plants that one who views them for the first time is struck with surprise, and might readily exclaim: "Here in the seventeenth century we have the foundation of the cell-theory." But these drawings were merely faithful representations of the appearance of the fabric of plants; the cells were not thought of as uniform elements of organic architecture, and no theory resulted. It is true that Malpighi understood that the cells were separable "utricles," and that plant tissue was the result of their union, but this was only an initial step in the direction of the cell-theory, which, as we shall see later, was founded on the supposed identity in development of cells in animals and plants. Fig. 73 shows a sketch, made by Malpighi about 1670, illustrating the microscopic structure of a plant. This is similar to the many drawings of Grew and Leeuwenhoek illustrating the structure of plant tissues. Wolff.--Nearly a century after the work of Malpighi, we find Wolff, in 1759, proposing a theory regarding the organization of animals and plants based upon observations of their mode of development. He was one of the most acute scientific observers of the period, and it is to be noted that his conclusions regarding structure were all founded upon what he was able to see; while he gives some theoretical conclusions of a purely speculative nature, Wolff was careful to keep these separate from his observations. The purpose of his investigations was to show that there was no pre-formation in the embryo; but in getting at the basis of this question, he worked out the identity of structure of plants and animals as shown by their development. In his famous publication on the Theory of Development (_Theoria Generationis_) he used both plants and animals. Huxley epitomizes Wolff's views on the development of elementary parts as follows: "Every organ, he says, is composed at first of a little mass of clear, viscous, nutritive fluid, which possesses no organization of any kind, but is at most composed of globules. In this semifluid mass cavities (_Bläschen_, _Zellen_) are now developed; these, if they remain round or polygonal, become the subsequent cells; if they elongate, the vessels; and the process is identically the same, whether it is examined in the vegetating point of a plant, or in the young budding organs of an animal."