The Evolution of Man, V.2, Ernst Haeckel [free ebook reader for pc txt] 📗

canals, which lead from the outer surface of the forehead into the rudimentary mouth. This transitory condition resembles that in which we find the nose permanently in the Dipneusts and Amphibia.

A cone-shaped structure, which grows from below towards the lower ends of the two nasal processes and joins with them, plays an important part in the conversion of the open nasal groove into the closed canal. This is the upper-jaw process (Figures 2.306 to 2.310 o). Below the mouth-pit are the gill-arches, which are separated by the gill-clefts. The first of these gill-arches, and the most important for our purpose, which we may call the maxillary (jaw) arch, forms the skeleton of the jaws. Above at the basis a small process grows out of this first gill-arch; this is the upper-jaw process. The first gill-arch itself develops a cartilage at one of its inner sides, the "Meckel cartilage" (named after its discoverer), on the outer surface of which the lower jaw is formed (Figures 2.306 to 2.310 u). The upper-jaw process forms the chief part of the skeleton of that jaw, the palate bone, and the pterygoid bone. On its outer side is afterwards formed the upper-jaw bone, in the narrower sense, while the middle part of the skeleton of the upper jaw, the intermaxillary, develops from the foremost part of the frontal process.

The two upper-jaw processes are of great importance in the further development of the face. From them is formed, growing into the primitive mouth-cavity, the important horizontal partition (the palate) that divides the former into two distinct cavities. The upper cavity, into which the nasal canals open, now develops into the nasal cavity, the air-passage and the organ of smell. The lower cavity forms the permanent secondary mouth (Figure 2.312 m), the food-passage and the organ of taste. Both the upper and lower cavities open behind into the gullet (pharynx). The hard palate that separates them is formed by the joining of two lateral halves, the horizontal plates of the two upper-jaw processes, or the palate-plates (p). When these do not, sometimes, completely join in the middle, a longitudinal cleft remains, through which we can penetrate from the mouth straight into the nasal cavity. This is the malformation known as "wolf's throat." "Hare-lip" is the lesser form of the same defect. At the same time as the horizontal partition of the hard palate a vertical partition is formed by which the single nasal cavity is divided into two sections--a right and left half (Figure 2.312 n, n).

(FIGURES 2.313 AND 2.314. Upper part of the body of a human embryo, two-thirds of an inch long, of the sixth week; Figure 2.313 from the left, Figure 2.314 from the front. The origin of the nose and the upper lip from two lateral and originally separate halves can be clearly seen. Nose and upper lip are large in proportion to the rest of the face, and especially to the lower lip. (From Kollmann.))

The double nose has now acquired the characteristic form that man shares with the other mammals. Its further development is easy to follow; it consists of the formation of the inner and outer processes of the walls of the two cavities. The external nose is not formed until long after all these essential parts of the internal organ of smell. The first traces of it in the human embryo are found about the middle of the second month (Figures 2.313 to 2.316). As can be seen in any human embryo during the first month, there is at first no trace of the external nose. It only develops afterwards from the foremost nasal part of the primitive skull, growing forwards from behind. The characteristic human nose is formed very late. Much stress is at times laid on this organ as an exclusive privilege of man. But there are apes that have similar noses, such as the long-nosed ape.

(FIGURE 2.315. Face of a human embryo, seven weeks old, (From Kollmann.) Joining of the nasal processes (e outer, i inner) with the upper-jaw process (o), n nasal wall, a ear-opening.)

The evolution of the eye is not less interesting and instructive than that of the nose. Although this noblest of the sensory organs is one of the most elaborate and purposive on account of its optic perfection and remarkable structure, it nevertheless develops, without preconceived design, from a simple process of the outer germinal layer. The fully-formed human eye is a round capsule, the eye-ball (Figure 2.317). This lies in the bony cavity of the skull, surrounded by protective fat and motor muscles. The greater part of it is taken up with a semi-fluid, transparent gelatinous substance, the corpus vitreum. The crystalline lens is fitted into the anterior surface of the ball (Figure 2.317 l). It is a lenticular, bi-convex, transparent body, the most important of the refractive media in the eye. Of this group we have, besides the corpus vitreum and the lens, the watery fluid (humor aqueus) that is found in front of the lens (at the letter m in Figure 2.317). These three transparent refractive media, by which the rays of light that enter the eye are broken up and re-focussed, are enclosed in a solid round capsule, composed of several different coats, something like the concentric layers of an onion. The outermost and thickest of these envelopes is the white sclerotic coat of the eye. It consists of tough white connective tissue. In front of the lens a circular, strongly-curved, transparent plate is fitted into the sclerotic, like the glass of a watch--the cornea (b). At its outer surface the cornea is covered with a very thin layer of the epidermis; this is known as the conjunctiva. It goes from the cornea over the inner surface of the eye-lids, the upper and lower folds which we draw over the eye in closing it. At the inner corner of the eye we have a rudimentary organ in the shape of the relic of a third (inner) eye-lid, which is greatly developed, as "nictitating (winking) membrane," in the lower Vertebrates (

Chapter 1.

5). Underneath the upper eye-lid are the lachrymal glands, the product of which, the lachrymal fluid, keeps the outer surface of the eye smooth and clean.

Immediately under the sclerotic we find a very delicate, dark-red membrane, very rich in blood-vessels--the choroid coat--and inside this the retina (o), the expansion of the optic nerve (i). The latter is the second cerebral nerve. It proceeds from the optic thalami (the second cerebral vesicle) to the eye; penetrates its outer envelopes, and then spreads out like a net between the choroid and the corpus vitreum. Between the retina and the choroid there is a very delicate membrane, which is usually (but wrongly) associated with the latter. This is the black pigment-membrane (n). It consists of a single stratum of graceful, hexagonal, regularly-joined cells, full of granules of black colouring matter. This pigment membrane clothes, not only the inner surface of the choroid proper, but also the hind surface of its anterior muscular continuation, which covers the edge of the lens in front as a circular membrane, and arrests the rays of light at the sides. This is the well-known iris of the eye (h), coloured differently in different individuals (blue, grey, brown, etc.); it forms the anterior border of the choroid. The circular opening that is left in the middle is the pupil, through which the rays of light penetrate into the eye. At the point where the iris leaves the anterior border of the choroid proper the latter is very thick, and forms a delicate crown of folds (g), which surrounds the edge of the lens with about seventy large and many smaller rays (corona ciliaris.)

At a very early stage a couple of pear-shaped vesicles develop from the foremost part of the first cerebral vesicle in the embryo of man and the other Craniotes (Figures 1.155 a and 2.297 au). These growths are the primary optic vesicles. They are at first directed outwards and forwards, but presently grow downward, so that, after the complete separation of the five cerebral vesicles, they lie at the base of the intermediate brain. The inner cavities of these pear-shaped vesicles, which soon attain a considerable size, are openly connected with the ventricle of the intermediate brain by their hollow stems. They are covered externally by the epidermis.

(FIGURE 2.316. Face of a human embryo, eight weeks old (From Ecker.))

At the point where this comes into direct contact with the most curved part of the primary optic vesicle there is a thickening (l) and also a depression (o) of the horny plate (Figure 2.318, I). This pit, which we may call the lens-pit, is converted into a closed sac, the thick-walled lens-vesicle (2, l), the thick edges of the pit joining together above it. In the same way in which the medullary tube separates from the outer germinal layer, we now see this lens-sac sever itself entirely from the horny plate (h), its source of origin. The hollow of the sac is afterwards filled with the cells of its thick walls, and thus we get the solid crystalline lens. This is, therefore, a purely epidermic structure. Together with the lens the small underlying piece of corium-plate also separates from the skin.

As the lens separates from the corneous plate and grows inwards, it necessarily hollows out the contiguous primary optic vesicle (Figure 2.318, 1 to 3). This is done in just the same way as the invagination of the blastula, which gives rise to the gastrula in the amphioxus (Figure 2.38 C to F). In both cases the hollowing of the closed vesicle on one side goes so far that at last the inner, folded part touches the outer, not folded part, and the cavity disappears. As in the gastrula the first part is converted into the entoderm and the latter into the ectoderm, so in the invagination of the primary optic vesicle the retina (r) is formed from the first (inner) part, and the black pigment membrane (u) from the latter (outer, non-invaginated) part. The hollow stem of the primary optic vesicle is converted into the optic nerve. The lens (l), which has so important a part in this process, lies at first directly on the invaginated part, or the retina (r). But they soon separate, a new structure, the corpus vitreum (gl), growing between them. While the lenticular sac is being detached and is causing the invagination of the primary optic vesicle, another invagination is taking place from below; this proceeds from the superficial part of the skin-fibre layer--the corium of the head. Behind and under the lens a last-shaped process rises from the cutis-plate (Figure 2.319 g), hollows out the cup-shaped optic vesicle from below, and presses between the lens (l) and the retina (i). In this way the optic vesicle acquires the form of a hood.

(FIGURE 2.317. The human eye in section. a sclerotic coat, b cornea, c conjunctiva, d circular veins of the iris, e choroid coat, f ciliary muscle, g corona ciliaris, h iris, i optic nerve, k anterior border of the retina, l crystalline lens, m inner covering of the cornea (aqueous membrane), n pigment membrane, o retina, p Petit's canal, q yellow spot of the retina. (From Helmholtz.))

Finally, a complete fibrous envelope, the fibrous capsule of the eye-ball, is formed about the secondary optic vesicle and its stem (the secondary optic nerve). It originates from the part of the head-plates which immediately encloses the eye. This fibrous envelope takes the form of a closed round vesicle, surrounding the whole of the ball and pushing between the lens and the horny plate at its outer side. The round wall of the capsule soon divides into two different membranes by surface-cleavage. The inner membrane becomes the choroid or vascular coat, and in front the ciliary corona and iris. The outer membrane is converted into the white protective or sclerotic coat--in front, the transparent cornea. The eye is now formed in all its