Let us pursue the individual development of the vertebrate animal body a few stages further, and see what is next done by the citizens of this embryonic organism. In the central line of the violin-shaped disc, which is composed of the three cellular germ-layers, there arises a straight delicate furrow, the so-called “primitive streak,” by which the violin-shaped body is divided into two equal lateral halves—a right and a left part or “antimer.” On both sides of that streak or furrow, the upper or external germ-layer rises in the form of a longitudinal fold, and both folds then grow together over the furrow in the central line, and thus form a cylindrical tube. This tube is called the marrow-tube, or medullary canal, because it is the foundation of the central nervous system, the spinal marrow (medulla spinalis). At first it is pointed both in front and behind, and it remains so for life in the lowest vertebrate animal, the brainless, skull-less Lancelet (Amphioxus). But in all other vertebrate animals, which we distinguish from the latter as skulled animals, or Craniota, a difference between the fore and hinder end of the marrow tube soon becomes visible, the fore end becoming dilated, and changing into a roundish bladder, the foundation of the brain.
In all Craniota, that is, in all vertebrate animals possessing skull and brain, the brain, which is at first only the bladder-shaped dilatation of the anterior end of the spinal marrow, divides into five bladders lying one behind the other, four superficial, transverse in-nippings being formed. These five brain-bladders, out of which afterwards arise all the different parts of the intricately constructed brain, can be seen in their original condition in the embryo represented in Fig. 7. It is just the same whether we examine the embryo of a dog, a fowl, a lizard, or any other higher vertebrate animal. For the embryos of the different skulled animals (at least the three higher classes of them, the reptiles, birds and mammals) cannot be in any way distinguished at the stage represented in Fig. 7. The whole form of the body is as yet exceedingly simple, being merely a thin, leaf-like disc. Face, legs, intestines, etc., are as yet completely wanting. But the five bladders are already quite distinct from one another.
Fig. 7.—Embryo of a mammal or bird, in which the five brain-bladders have just commenced to develop. v. Fore brain. z. Twixt brain. m. Mid brain. h. Hind brain. n. After brain. p. Spinal-marrow. a. Eye-bladders. w. Primitive vertebræ. d. Spinal-axis or notochord.
The first bladder, the fore brain (a), is in so far the most important that it principally forms the hemispheres of the so-called larger brain (cerebrum), that part which is the seat of the higher mental activities. The more these activities are developed in the series of vertebrate animals, the more do the two lateral halves of the fore brain, or the hemispheres, grow at the expense of the other bladders, and overlap them in front and from above. In man, where they are most strongly developed, agreeing with his higher mental activity, they eventually almost entirely cover the other parts from above (compare Plates II. and III.) The second bladder, the twixt brain (z), forms that portion of the brain which is called the centre of sight, and stands in the closest relation to the eyes (a), which grow right and left out of the fore brain in the shape of two bladders, and later lie at the bottom of the twixt brain. The third bladder, the mid brain (m), for the most part vanishes in the formation of the so-called four bulbs, a bossy portion of the brain, which is strongly developed in reptiles and birds (Fig. E, F, Plate II.), whereas in mammals it recedes 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 (F bv), 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 (H bh), of a bird (F bh), the hind-leg of a dog (G bh), and that of a tortoise (E bh). The fore as well as the hinder extremities are as yet short, broad lumps, at the ends of which the foundations of the five toes are placed, connected as yet by a membrane. At a still earlier stage (Fig. A-D) the five toes are not marked out at all, and it is quite impossible to distinguish even the fore and hinder extremities from one another. The latter, as well as the former, are nothing but simple roundish processes, which have grown out of the side of the trunk. At the very early stage represented in Fig. 7 they are completely wanting, and the whole embryo is a simple trunk without a trace of limbs.
Germs or Embryos of four Vertebrates.
Pl. II.