The Evolution of Man, V.2 by Ernst Haeckel (comprehension books .TXT) 📕

But before we consider the development of the complicated structure of the brain from this simple series of vesicles, let us glance for a moment at the lower animals, which have no brain. Even in the skull-less vertebrate, the Amphioxus, we find no independent brain, as we have seen. The whole central marrow is merely a simple cylindrical cord which runs the length of the body, and ends equally simply at both extremities--a plain medullary tube. All that we can discover is a small vesicular bulb at the foremost part of the tube, a degenerate rudiment of a primitive brain. We meet the same simple medullary tube in the first structure of the ascidia larva, in the same characteristic position, above the chorda. On closer examination we find here also a small vesicular swelling at the fore end of the tube, the first trace of a differentiation of it into brain and spinal cord. It is probable that this differentiation was more advanced in the extinct Provertebrates, and the brain-bulb more pronounced (Figures 1.98 to 1.102). The brain is phylogenetically older than the spinal cord, as the trunk was not developed until after the head. If we consider the undeniable affinity of the Ascidiae to the Vermalia, and remember that we can trace all the Chordonia to lower Vermalia, it seems probable that the simple central marrow of the former is equivalent to the simple nervous ganglion, which lies above the gullet in the lower worms, and has long been known as the "upper pharyngeal ganglion" (ganglion pharyngeum superius); it would be better to call it the primitive or vertical brain (acroganglion).

Probably this upper pharyngeal ganglion of the lower worms is the structure from which the complex central marrow of the higher animals has been evolved. The medullary tube of the Chordonia has been formed by the lengthening of the vertical brain on the dorsal side. In all the other animals the central nervous system has been developed in a totally different way from the upper pharyngeal ganglion; in the Articulates, especially, a pharyngeal ring, with ventral marrow, has been added. The Molluscs also have a pharyngeal ring, but it is not found in the Vertebrates. In these the central marrow has been prolonged down the dorsal side; in the Articulates down the ventral side. This fact proves of itself that there is no direct relationship between the Vertebrates and the Articulates. The unfortunate attempts to derive the dorsal marrow of the former from the ventral marrow of the latter have totally failed (cf.

Chapter 2.

20).

(FIGURE 2.293. The human brain, seen from the left. (From H. Meyer.) The furrows of the cerebrum are indicated by thick, and those of the cerebellum by finer lines. Under the latter we can see the medulla oblongata. f1 to f2 frontal convolutions, C central convolutions, S fissure of Sylvius, T temporal furrow, Pa parietal lobes, An angular gyrus, Po parieto-occipital fissure.)

When we examine the embryology of the human nervous system, we must start from the important fact, which we have already seen, that the first structure of it in man and all the higher Vertebrates is the simple medullary tube, and that this separates from the outer germinal layer in the middle line of the sole-shaped embryonic shield. As the reader will remember, the straight medullary furrow first appears in the middle of the sandal-shaped embryonic shield. At each side of it the parallel borders curve over in the form of dorsal or medullary swellings. These bend together with their free borders, and thus form the closed medullary tube (Figures 1.133 to 1.137). At first this tube lies directly underneath the horny plate; but it afterwards travels inwards, the upper edges of the provertebral plates growing together between the horny plate and the tube, joining above the latter, and forming a completely closed canal. As Gegenbaur very properly observes, "this gradual imbedding in the inner part of the body is a process acquired with the progressive differentiation and the higher potentiality that this secures; by this process the organ of greater value to the organism is buried within the frame." (Cf. Figures 1.143 to 1.146).

(FIGURES 2.294 TO 2.296. Central marrow of the human embryo from the seventh week, 4/5 inch long. (From Kolliker.)

FIGURE 2.294. The brain from above, v fore brain, z intermediate brain, m middle brain, h hind brain, n after brain.

FIGURE 2.295. The brain with the uppermost part of the cord, from the left.

FIGURE 2.296. Back view of the whole embryo: brain and spinal cord exposed.)

In the Cyclostoma--a stage above the Acrania--the fore end of the cylindrical medullary tube begins early to expand into a pear-shaped vesicle; this is the first outline of an independent brain. In this way the central marrow of the Vertebrates divides clearly into its two chief sections, brain and spinal cord. The simple vesicular form of the brain, which persists for some time in the Cyclostoma, is found also at first in all the higher Vertebrates (Figure 1.153 hb). But in these it soon passes away, the one vesicle being divided into several successive parts by transverse constrictions. There are first two of these constrictions, dividing the brain into three consecutive vesicles (fore brain, middle brain, and hind brain, Figure 1.154 v, m, h). Then the first and third are sub-divided by fresh constrictions, and thus we get five successive sections (Figure 1.155).

In all the Craniotes, from the Cyclostoma up to man, the same parts develop from these five original cerebral vesicles, though in very different ways. The first vesicle, the fore brain (Figure 1.155 v), forms by far the largest part of the cerebrum--namely, the large hemispheres, the olfactory lobes, the corpora striata, the callosum, and the fornix. From the second vesicle, the intermediate brain (z), originate especially the optic thalami, the other parts that surround the third cerebral ventricle, and the infundibulum and pineal gland. The third vesicle, the middle brain (m), produces the corpora quadrigemina and the aqueduct of Sylvius. From the fourth vesicle, the hind brain (h), develops the greater part of the cerebellum--namely, the vermis and the two small hemispheres. Finally, the fifth vesicle, the after brain (n), forms the medulla oblongata, with the quadrangular pit (the floor of the fourth ventricle), the pyramids, olivary bodies, etc.

We must certainly regard it as a comparative-anatomical and ontogenetic fact of the greatest significance that in all the Craniotes, from the lowest Cyclostomes and fishes up to the apes and man, the brain develops in just the same way in the embryo. The first rudiment of it is always a simple vesicular enlargement of the fore end of the medullary tube. In every case, first three, then five, vesicles develop from this bulb, and the permanent brain with all its complex anatomic structures, of so great a variety in the various classes of Vertebrates, is formed from the five primitive vesicles. When we compare the mature brain of a fish, an amphibian, a reptile, a bird, and a mammal, it seems incredible that we can trace the various parts of these organs, that differ so much internally and externally, to common types. Yet all these different Craniote brains have started with the same rudimentary structure. To convince ourselves of this we have only to compare the corresponding stages of development of the embryos of these different animals.

(FIGURE 2.297. Head of a chick embryo (hatched fifty-eight hours), from the back, magnified forty times. (From Mihalkovics.) vw anterior wall of the fore brain. vh its ventricle. au optic vesicles, mh middle brain, kh hind brain, nh after brain, hz heart (seen from below), vw vitelline veins, us primitive segment, rm spinal cord.)

This comparison is extremely instructive. If we extend it through the whole series of the Craniotes, we soon discover this interesting fact: In the Cyclostomes (the Myxinoida and Petromyzonta), which we have recognised as the lowest and earliest Craniotes, the whole brain remains throughout life at a very low stage, which is very brief and passing in the embryos of the higher Craniotes; they retain the five original sections of the brain unchanged. In the fishes we find an essential and considerable modification of the five vesicles; it is clearly the brain of the Selachii in the first place, and subsequently the brain of the Ganoids, from which the brain of the rest of the fishes on the one hand and of the Dipneusts and Amphibia, and through these of the higher Vertebrates, on the other hand, must be derived. In the fishes and Amphibia (Figure 2.300) there is a preponderant development of the middle brain, and also the after brain, the first, second, and fourth sections remaining very primitive. It is just the reverse in the higher Vertebrates, in which the first and third sections, the cerebrum and cerebellum, are exceptionally developed; while the middle brain and after brain remain small. The corpora quadrigemina are mostly covered by the cerebrum, and the oblongata by the cerebellum. But we find a number of stages of development within the higher Vertebrates themselves. From the Amphibia upwards the brain (and with it the psychic life) develops in two different directions; one of these is followed by the reptiles and birds, and the other by the mammals. The development of the first section, the fore brain, is particularly characteristic of the mammals. It is only in them that the cerebrum becomes so large as to cover all the other parts of the brain (Figures 2.293 and 2.301 to 2.304).

There are also notable variations in the relative position of the cerebral vesicles. In the lower Craniotes they lie originally almost in the same plane. When we examine the brain laterally, we can cut through all five vesicles with a straight line. But in the Amniotes there is a considerable curve in the brain along with the bending of the head and neck; the whole of the upper dorsal surface of the brain develops much more than the under ventral surface. This causes a curve, so that the parts come to lie as follows: The fore brain is right in front and below, the intermediate brain a little higher, and the middle brain highest of all; the hind brain lies a little lower, and the after brain lower still. We find this only in the Amniotes--the reptiles, birds, and mammals.

(FIGURE 2.298. Brain of three craniote embryos in vertical section. A of a shark (Heptarchus), B of a serpent (Coluber), C of a goat (Capra). a fore brain, b intermediate brain, c middle brain, d hind brain, e after brain, s primitive cleft. (From Gegenbaur.)

FIGURE 2.299. Brain of a shark (Scyllium), back view. g fore-brain, h olfactory lobes, which send the large olfactory nerves to the nasal capsule (o), d intermediate brain, b middle brain; behind this the insignificant structure of the hind brain, a after brain. (From Gegenbaur.)

FIGURE 2.300. Brain and spinal cord of the frog. A from the dorsal, B from the ventral side. a olfactory lobes before the (b) fore brain, i infundibulum at the base of the intermediate brain, c middle brain, d hind brain, s quadrangular pit in the after brain, m spinal cord (very short in the frog), m apostrophe roots of the spinal nerves, t terminal fibres of the spinal cord. (From Gegenbaur.)

FIGURE 2.301. Brain of an ox-embryo, two inches in length. (From Mihalkovics, magnified three times.) Left view; the lateral wall of the left hemisphere has been removed, st corpora striata, ml