Here, then, we leave the lower forms of Metazoa in their condition of permanent gastrulæ. They differ from the transitory stage of other Metazoa only in being enormously larger (owing to greatly further _growth_, without any further _development_ as to matters of fundamental importance), and in having sundry tentacles and other organs added later on to meet their special requirements. The point to remember is, that in all cases a gastrula is an open sac composed of two layers of cells--the outer layer being called the ectoderm, and the inner the endoderm. They have also been called the animal layer and the vegetative layer, because it is the outer layer (ectoderm) that gives rise to all the organs of sensation and movement--viz. the skin, the nervous system, and the muscular system; while it is the inner layer (endoderm) that gives rise to all the organs of nutrition and reproduction. It is desirable only further to explain that gastrulation does not take place in all the Metazoa after exactly the same plan. In different lines of descent various and often considerable modifications of the original and most simple plan have been introduced; but I will not burden the present exposition by describing these modifications[15]. It is enough for us that they always end in the formation of the two primary layers of ectoderm and endoderm. [15] The most extreme of them is that which is mentioned in the last foot-note. The next stage of differentiation is common to all the Metazoa, except those lowest forms which, as we have just seen, remain permanently as large gastrulæ, with sundry specialized additions in the way of tentacles, &c. This stage of differentiation consists in the formation of either a pouch or an additional layer between the ectoderm and the endoderm, which is called the mesoderm. It is probably in most cases derived from the endoderm, but the exact mode of its derivation is still somewhat obscure. Sometimes it has the appearance of itself constituting two layers; but it is needless to go into these details; for in any case the ultimate result is the same--viz. that of converting the Metazoön into the form of a tube, the walls of which are composed of concentric layers of cells. The outermost layer afterwards gives rise to the epidermis with its various appendages, and also to the central nervous system with its organs of special sense. The median layer gives rise to the voluntary muscles, bones, cartilages, &c., the nutritive systems of the blood, the chyle, the lymph, and the muscular tube of the intestine. Lastly, the innermost layer developes into the epithelium lining of the intestine, with its various appendages of liver, lungs, intestinal glands, &c.