§ 289. This account of the primary differentiation carries us only half-way towards a true conception of the distinction between outer and inner tissues. Though, using words in their current senses, this introverted part of the primitive layer has become internal in contrast with the remainder, which continues external, yet this introverted part has not become internal in the strict physiological sense. For it remains subject to the actions of those environing matters which are taken in as food: such environing matters, when they happen to be moving prey, acting upon it much as they might act upon the exterior. So that this introverted part has a quasi-externality. It has not the same absolute internality as have those parts which never come in contact with products of the outer world. Here we must briefly recognize the distinction between these parts and the parts thus far considered. Reverting to our symbol, the india-rubber ball, it will be seen that the introversion may be so complete that the cavity is obliterated, with the result that the internal surfaces of the outer and inner layers come in contact. This is the state reached in the simplest cœlenterate animal, the _Hydra_: there being in it nothing more than a thin structureless lamella between the ectoderm and endoderm, as shown in Fig. 147. This lamella represents all that there is of strictly internal tissues. But the introversion, instead of bringing the inner surfaces of the ball into contact, may be so far incomplete as to leave a space, and in various creatures and embryos of others, symbolized by this arrangement, this space becomes occupied by a tissue formed from one or other or both of the two primary tissues--the mesoblast or mesoderm. This intermediate layer, sometimes, as in the _Medusa_, growing into a mass of jelly serving as a fulcrum for the creature’s contractions, or, as in the Sponge, giving a passive basis to the active tissues, becomes in higher animals the layer out of which the structures that support the body and move it about, as well as those that distribute prepared nutriment, are developed. From it arise the bones, the muscles, and the vascular system--the masses of differentiated tissue which are truly internal and occupy what is called the body-cavity or peri-visceral space.