Basic principles of cellular organization.
The hypothesis is put forward that the division of the animal body into three germ layers represents not only histological, but also functional specialization, the ectoderm taking over functions through which the animal is in contact with, acted upon or reacts to the external world, the entoderm metabolism and the mesoderm architectural and mechanical organization. This is assumed to mean that the tasks of life have been divided into three large function domains at an early stage of multicellular evolution, each germ layer taking over one domain and developing largely independently of the other two in the further course of phylogeny. The germ layers are autonomous in their own spheres, giving the body, in effect, a tripartite government. The potentiality of conceptual thought is inherent in all germ layers, reaching a high state of development in the entoderm and mesoderm at a much earlier stage of development than in the ectoderm. The animal body, as a whole, is in control of all its biologic activities; it is a self-designing, self-developing and self-perpetuating entity. The control of cellular organization is a mesodermal task.