THE FIVE BLADDERS OF THE BRAIN.

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much more (Fig. G, H, Plate III.). The fourth bladder, the hind brain (h), forms the so-called little hemispheres, together with the middle part of the small brain (cerebellum), a part of the brain as to the function of which the most contradictory conjectures are entertained, but which seems principally to regulate the co-ordination of movements. Lastly, the fifth bladder, the after brain (n), develops into that very important part of the central nervous system which is called the prolonged marrow (medulla oblongata). It is the central organ of the respiratory movements, and of other important functions, and an injury to it immediately causes death, whereas the large hemispheres of the fore brain (or the organ of the "soul," in a restricted sense) can be removed bit by bit, and even completely destroyed, without causing the death of the vertebrate animal-only its higher mental activities disappearing in consequence.

These five brain bladders, in all vertebrate animals which possess a brain at all, are originally arranged in the same manner and develop gradually in the different groups so differently, that it is afterwards very difficult to recognize the corresponding parts in the fully-developed brains. In the early stage of development which is represented in Fig. 7, it seems as yet quite impossible to distinguish the embryos of the different mammals, birds, and reptiles, from one another. But if we compare the much more developed embryos on Plates II. and III. with one another, we can clearly see an inequality in their development, and especially it will be perceived that the brain of the two mammals. (G and H) already strongly differ from that of birds (F) and of reptiles (E). In the two latter the mid brain predominates, but in the former the fore brain. Even at this stage the

brain of the bird (F) is scarcely distinguishable from that of the tortoise (E), and in like manner the brain of the dog (G) is as yet almost the same as that of man (H). If, on the other hand, we compare the brains of these four vertebrate animals in a fully developed condition, we find them so very different in all anatomical particulars, that we cannot doubt for a moment as to which animal each brain belongs.

I have here explained the original equality, the gradual commencement, and the ever increasing separation or differentiation of the embryos in the different vertebrate animals, taking the brain as a special example, just because this organ of the soul's activity is of special interest. But I might as well have discussed in its stead the heart, or the liver, or the limbs, in short, any other part of the body, since the same wonder of creation is here ever repeated, namely, this, that all parts are originally the same in the different vertebrate animals, and that the. variations by which the different classes, orders, families, genera, etc., differ and deviate from one another, are only gradually developed.

There are certainly few parts of the body which are so differently constructed as the limbs or extremities of the vertebrate animals. Now, I wish the reader to compare in Fig. A-H on Plates II. and III., the four extremities (bv) of the embryos with one another, and he will scarcely be able to perceive any important differences between the human arm (H bv), the wing of a bird (Fbv), the slim foreleg of a dog (G bv), and the plump foreleg of the tortoise (E bv). In comparing the hinder extremities (bh) in these figures he will find it equally difficult to distinguish the leg of a man (Hbh), of a bird (Fbh), the hind-leg of a dog (Gbh), and that of a tortoise (Ebh). The fore as well as the hinder