As we know from previous chapters (1.11 to 1.13), the animal systems of organs (the organs of sensation and presentation) develop for the most part out of the OUTER primary germ-layer, or the cutaneous (skin) layer. On the other hand, the vegetal systems of organs arise for the most part from the INNER primary germ-layer, the visceral layer. It is true that this antithesis of the animal and vegetal spheres of the body in man and all the higher animals is by no means rigid; several parts of the animal apparatus (for instance, the greater part of the muscles) are formed from cells that come originally from the entoderm; and a great part of the vegetative apparatus (for instance, the mouth-cavity and the gonoducts) are composed of cells that come from the ectoderm. In the more advanced animal body there is so much interlacing and displacement of the various parts that it is often very difficult to indicate the sources of them. But, broadly speaking, we may take it as a positive and important fact that in man and the higher animals the chief part of the animal organs comes from the ectoderm, and the greater part of the vegetative organs from the entoderm. It was for this reason that Carl Ernst von Baer called the one the animal and the other the vegetative layer (see Chapter 1.3). The solid foundation of this important thesis is the gastrula, the most instructive embryonic form in the animal world, which we still find in the same shape in the most diverse classes of animals. This form points demonstrably to a common stem-form of all the Metazoa, the Gastraea; in this long-extinct stem-form the whole body consisted throughout life of the two primary germinal layers, as is now the case temporarily in the gastrula; in the Gastraea the simple cutaneous (skin) layer ACTUALLY represented all the animal organs and functions, and the simple visceral (gut) layer all the vegetal organs and functions. This is the case with the modern Gastraeads (Figure 2.233); and it is also the case potentially with the gastrula.