File:Plate IV. Diagrammatic transverse sections of different embryos.jpg

Plate IV.—DIAGRAMMATIC TRANSVERSE SECTIONS.

Fig. 1.—Transverse section through the Gastrula. (Compare Fig. 9, longitudinal section, and Figs. 22-28, p. 193.) The whole body is formed by the intestinal tube (d); the wall of this consists solely of the two primary germ-layers.

Fig. 2.—Transverse section through the larva of the Amphioxus, in the early stage in which the body consists merely of the four secondary germlayers. The intestinal tube (d), formed of the intestinal layer, is separated from the body-wall by the coelom (c), which is formed of the skin-layer.

Fig. 3.—Transverse section through the germ-disc of a higher Vertebrate, with the rudiments of the earliest organs. (Compare the transverse section of the embryo Chick at the second day of incubation, Fig. 92.) The spinal tube (m) and the primitive kidneys (u) are separated from the horn-plate (h). On both sides of the notochord (ch) the primitive vertebrae (ww) and the side-layers are differentiated. Between the skin-fibrous layer and the intes.separated from the horn-plate (fc). On both sides of the notochord (ch) the primitive vertebrae (uw) and the side-layers are differentiated. Between the skin-fibrous layer and the intestinal-fibrous layer, the first rudiment of the body-cavity, or the coelom (e), in visible; under it are the two primitive aortas (t).

Fig. 4.—Transverse section through the germe-disc of a higher Vertebrate, somewhat further developed than in Fig. 3. (Compare the transverse section of the embryo Chick at the third day of incubation, Fig. 95 and 96, p- 317.) The spinal tube (m) and the notochord (ch) are already beginning to be enclosed by the primitive vertebrae (uw), in which the muscle-plates, bone-plates, and nerve-roots are becoming distinct. The primitive kidneys (u) are already completely separated from the horn-plate (h) by the leather plate (l); c, the coelom; t, the aortas. The skin-layer, rising around the embryo, forms the amnion-fold (am); this gives rise to a hollow space (g) between the amnion-fold and the wall of the yelk-sac (ds).

Fig. 5. —Transverse section through the pelvic region and the posterior limbs of the embryo of a higher Vertebrate. (Compare the transverse section through Chick at the fifth day of incubation, Fig. 120.) The spinal tube (m) is already entirely enclosed by the two curving halves of the vertebrae (wb), and similarly the notochord and its sheath by the two halves of the vertebral body (wk). The leather-plate (l) has entirely separated from the muscle-plate (mp). The horn-plate (hk) has thickened very much at the head of the posterior limbs (x). The primitive kidneys (u) are prominent in the coelom (c), and lie very near the germ-epithelium, or the rudimentary sexual glands (k). The intestinal tube (d) is attached to the dorsal surface of the body by the mesentery (g), beneath the main artery (t), and the two principal veins (n). Below, in the centre of the ventral wall, the stalk of the allantois (al) is visible.

Fig. 6.—Transverse section through a developed Primitive Fish, or some other Vertebrate of a low order. The parts, on the whole, bear the same relation to each other as in the preceding transverse section, Fig. 5, and are marked in the same way. But the sexual glands (k) have developed into ovaries, and the primitive kidneys are transferred into oviducts, which open into the coelom. The two side protuberances (1b) of the intestinal tube (d) indicate the intestinal glands, for example, the liver. Below the intestinal tube, in the intestinal wall, lies the intestinal vein (v); above the intestinal tube lies the aorta (t), and above this, again, the two principal veins (n).

Fig. 7.—Transverse section through one of the higher Worms (through the head of an Annelid), showing its essential agreement with the Vertebrates in the construction of the body from the four secondary germ. layers. It should be carefully compared with the diagrammatic transverse section through the low Vertebrate, Fig. 6: m, the “ brain,” or “upper throat ganglion.” The leather-plate (l) and the muscle-plate, which lies below the former, have differentiated from the skin-fibrous layer. The muscle-layer has separated into an outer circular muscle-stratum and a long inner stratum, and the muscle of the latter has distributed itself into dorsal muscles (r) and ventral muscles (b). The two are separated by the primitive kidneys (u), which extend from the horn-plate (hk) to the coelom (c). Here the primitive: kidneys have a funnel-shaped opening, through which they carry out the ovules, which fall from the ovaries (k) into the coelom. The intestinal tube (d) has glands on its surface (liver-vesicles, lb). Below it lies the ventral vessel (the intestinal vein, v), above it the dorsal vessel (the aorta, t). The position and origin of all these primitive organs is entirely the same in Mari and every other Vertebrate, as in the Worms. The only essential difference is that in the Vertebrates a notochord is developed between the spinal tube and the intestinal tube.

Fig. 8.—Transverse section through the human thorax. The spinal tubo (m) is entirely enclosed by the developed circular vertebrae (w). A curved rib proceeds right and left from the vertebra, supporting the wall of the breast (rp). Below, on the ventral surface, between the right and left rib, lies the breast-bone, or sternum (db). Without, above the ribs, and the muscles between the ribs, lies the outer skin, formed from the leather-plate (l) and the horn-plate (hk). The greater part of the breast-cavity (or the anterior part of the coelom, c) is occupied by the two lungs (lu), in which the branches of the trachea ramify like a tree. These all open together into the unequal branches of the trachea (lr), which opens further up: at the neck into the oesophagus (sr). Between the intestinal tube and the vertebral column, lies the aorta (t). Between the trachea and the sternum lies the heart divided by a partition wall into two halves. The left heart (hl) contains only arterial, the right (hr) only venous blood. Each half of the heart is divided by a valved opening into an auricle and a ventricle. The heart is here represented diagrammatically in its (phylogenetic) original symmetrical position (in the centre of the ventral side). In the developed