The Evolution of Man, V.1. by Ernst Haeckel (books for 9th graders .txt) 📕

While these important processes are taking place in the axial part of the dorsal shield, its external form also is changing. The oval form (Figure 1.117) becomes like the sole of a shoe or sandal, lyre-shaped or finger-biscuit shaped (Figure 1.130). The middle third does not grow in width as quickly as the posterior, and still less than the anterior third; thus the shape of the permanent body becomes somewhat narrow at the waist. At the same time, the oval form of the germinative area returns to a circular shape, and the inner pellucid area separates more clearly from the opaque outer area (Figure 1.131 a). The completion of the circle in the area marks the limit of the formation of blood-vessels in the mesoderm.

(FIGURE 1.129. Longitudinal section of the hinder end of a chick. (From Balfour.) sp medullary tube, connected with the terminal gut (pag) by the neurenteric canal (ne), ch chorda, pr neurenteric (or Hensen's) ganglion, al allantois, ep ectoderm, hy entoderm, so parietal layer, sp visceral layer, an anus-pit, am amnion.)

The characteristic sandal-shape of the dorsal shield, which is determined by the narrowness of the middle part, and which is compared to a violin, lyre, or shoe-sole, persists for a long time in all the amniotes. All mammals, birds, and reptiles have substantially the same construction at this stage, and even for a longer or shorter period after the division of the primitive segments into the coelom-folds has begun (Figure 1.132). The human embryonic shield assumes the sandal-form in the second week of development; towards the end of the week our sole-shaped embryo has a length of about one-twelfth of an inch (Figure 1.133).

The complete bilateral symmetry of the vertebrate body is very early indicated in the oval form of the embryonic shield (Figure 1.117) by the median primitive streak; in the sandal-form it is even more pronounced (Figures 1.131 to 1.135). In the lateral parts of the embryonic shield a darker central and a lighter peripheral zone become more obvious; the former is called the stem-zone (Figure 1.134 stz), and the latter the parietal zone (pz); from the first we get the dorsal and from the second the ventral half of the body-wall. The stem-zone of the amniote embryo would be called more appropriately the dorsal zone or dorsal shield; from it develops the whole of the dorsal half of the later body (or permanent body)--that is to say, the dorsal body (episoma). Again, it would be better to call the "parietal zone" the ventral zone or ventral shield; from it develop the ventral "lateral plates," which afterwards separate from the embryonic vesicle and form the ventral body (hyposoma)--that is to say, the ventral half of the permanent body, together with the body-cavity and the gastric canal that it encloses.

(FIGURE 1.130. Germinal area or germinal disk of the rabbit, with sole-shaped embryonic shield, magnified about ten times. The clear circular field (d) is the opaque area. The pellucid area (c) is lyre-shaped, like the embryonic shield itself (b). In its axis is seen the dorsal furrow or medullary furrow (a). (From Bischoff.))

The sole-shaped germinal shields of all the amniotes are still, at the stage of construction which Figure 1.134 illustrates in the rabbit and Figure 1.135 in the opossum, so like each other that we can either not distinguish them at all or only by means of quite subordinate peculiarities in the size of the various parts. Moreover, the human sandal-shaped embryo cannot at this stage be distinguished from those of other mammals, and it particularly resembles that of the rabbit. On the other hand, the outer form of these flat sandal-shaped embryos is very different from the corresponding form of the lower animals, especially the acrania (amphioxus). Nevertheless, the body is just the same in the essential features of its structure as that we find in the chordula of the latter (Figures 1.83 to 1.86), and in the embryonic forms which immediately develop from it. The striking external difference is here again due to the fact that in the palingenetic embryos of the amphioxus (Figures 1.83 and 1.84) and the amphibia (Figures 1.85 and 1.86) the gut-wall and body-wall form closed tubes from the first, whereas in the cenogenetic embryos of the amniotes they are forced to expand leaf-wise on the surface owing to the great extension of the food-yelk.

(FIGURE 1.131. Embryo of the opossum, sixty hours old, one-sixth of an inch in diameter. (From Selenka) b the globular embryonic vesicle, a the round germinative area, b limit of the ventral plates, r dorsal shield, v its fore part, u the first primitive segment, ch chorda, chr its fore-end, pr primitive groove (or mouth).

FIGURE 1.132. Sandal-shaped embryonic shield of a rabbit of eight days, with the fore part of the germinative area (ao opaque, ap pellucid area). (From Kolliker.) rf dorsal furrow, in the middle of the medullary plate, h, pr primitive groove (mouth), stz dorsal (stem) zone, pz ventral (parietal) zone. In the narrow middle part the first three primitive segments may be seen.)

It is all the more notable that the early separation of dorsal and ventral halves takes place in the same rigidly hereditary fashion in all the vertebrates. In both the acrania and the craniota the dorsal body is about this period separated from the ventral body. In the middle part of the body this division has already taken place by the construction of the chorda between the dorsal nerve-tube and the ventral canal. But in the outer or lateral part of the body it is only brought about by the division of the coelom-pouches into two sections--a dorsal episomite (dorsal segment or provertebra) and a ventral hyposomite (or ventral segment) by a frontal constriction. In the amphioxus each of the former makes a muscular pouch, and each of the latter a sex-pouch or gonad.

These important processes of differentiation in the mesoderm, which we will consider more closely in the next chapter, proceed step by step with interesting changes in the ectoderm, while the entoderm changes little at first. We can study these processes best in transverse sections, made vertically to the surface through the sole-shaped embryonic shield. Such a transverse section of a chick embryo, at the end of the first day of incubation, shows the gut-gland layer as a very simple epithelium, which is spread like a leaf over the outer surface of the food-yelk (Figure 1.92). The chorda (ch) has separated from the dorsal middle line of the entoderm; to the right and left of it are the two halves of the mesoderm, or the two coelom-folds. A narrow cleft in the latter indicates the body-cavity (uwh); this separates the two plates of the coelom-pouches, the lower (visceral) and upper (parietal). The broad dorsal furrow (Rf) formed by the medullary plate (m) is still wide open, but is divided from the lateral horn-plate (h) by the parallel medullary swellings, which eventually close.

(FIGURE 1.133. Human embryo at the sandal-stage, one-twelfth of an inch long, from the end of the second week, magnified twenty-five times. (From Count Spee.)

FIGURE 1.134. Sandal-shaped embryonic shield of a rabbit of nine days. (From Kolliker.) (Back view from above.) stz stem-zone or dorsal shield (with eight pairs of primitive segments), pz parietal or ventral zone, ap pellucid area, af amnion-fold, h heart, ph pericardial cavity, vo omphalo-mesenteric vein, ab eye-vesicles, vh fore brain, mh middle brain, hh hind brain, uw primitive segments (or vertebrae).)

During these processes important changes are taking place in the outer germinal layer (the "skin-sense layer"). The continued rise and growth of the dorsal swellings causes their higher parts to bend together at their free borders, approach nearer and nearer (Figure 1.136 w), and finally unite. Thus in the end we get from the open dorsal furrow, the upper cleft of which becomes narrower and narrower, a closed cylindrical tube (Figure 1.137 mr). This tube is of the utmost importance; it is the beginning of the central nervous system, the brain and spinal marrow, the medullary tube. This embryonic fact was formerly looked upon as very mysterious. We shall see presently that in the light of the theory of descent it is a thoroughly natural process. The phylogenetic explanation of it is that the central nervous system is the organ by means of which all intercourse with the outer world, all psychic action and sense-perception, are accomplished; hence it was bound to develop originally from the outer and upper surface of the body, or from the outer skin. The medullary tube afterwards separates completely from the outer germinal layer, and is surrounded by the middle parts of the provertebrae and forced inwards (Figure 1.146). The remaining portion of the skin-sense layer (Figure 1.93 h) is now called the horn-plate or horn-layer, because from it is developed the whole of the outer skin or epidermis, with all its horny appendages (nails, hair, etc.).

(FIGURE 1.135. Sandal-shaped embryonic shield of an opossum (Didelphys), three days old. (From Selenka.) (Back view from above.) stz stem-zone or dorsal shield (with eight pairs of primitive segments), pz parietal or ventral zone, ap pellucid area, ao opaque area, hh halves of the heart, v fore-end, h hind-end. In the median line we see the chorda (ch) through the transparent medullary tube (m). u primitive segment, pr primitive streak (or primitive mouth).)

A totally different organ, the prorenal (primitive kidney) duct (ung), is found to be developed at an early stage from the ectoderm. This is originally a quite simple, tube-shaped, lengthy duct, or straight canal, which runs from front to rear at each side of the provertebrae (on the outer side, Figure 1.93 ung). It originates, it seems, out of the horn-plate at the side of the medullary tube, in the gap that we find between the provertebral and the lateral plates. The prorenal duct is visible in this gap even at the time of the severance of the medullary tube from the horn-plate. Other observers think that the first trace of it does not come from the skin-sense layer, but the skin-fibre layer.

The inner germinal layer, or the gut-fibre layer (Figure 1.93 dd), remains unchanged during these processes. A little later, however, it shows a quite flat, groove-like depression in the middle line of the embryonic shield, directly under the chorda. This depression is called the gastric groove or furrow. This at once indicates the future lot of this germinal layer. As this ventral groove gradually deepens, and its lower edges bend towards each other, it is formed into a closed tube, the alimentary canal, in the same way as the medullary groove grows into the medullary tube. The gut-fibre layer (Figure 1.137 f), which lies on the gut-gland layer (d), naturally follows it in its folding. Moreover, the incipient gut-wall consists from the first of two layers, internally the gut-gland layer and externally the gut-fibre layer.

The formation of the alimentary canal resembles that of the medullary tube to this extent--in both cases a straight groove or furrow arises first of all in the middle line of a flat layer. The edges of this furrow then bend towards each other, and join to form a tube (Figure 1.137). But the two processes are really very different. The medullary tube closes in its whole length, and forms a cylindrical tube, whereas the alimentary canal remains open in the middle, and its cavity continues for a long time in connection with the cavity of the embryonic vesicle. The open connection between the two cavities is only closed at a very late stage, by the construction of the navel. The closing of the