File:Plate V. Diagrammatic longitudinal sections of different embryos.jpg

Plate V.—DIAGRAMMATIC LONGITUDINAL SECTIONS,

Fig. 9.—Longitudinal section through a Gastrula. (Compare Fig. ?), transverse section.) The intestinal cavity (d) opens in front through the mouth (o). The body consists merely of the two primary germ-layers.

Fig. 10.—Longitudinal section through an hypothetical Primitive Worm (Prothelmis), the entire body of which consists of the four secondary germ-layers. The intestinal tube (d) is still very simple; but the anterior and posterior intestines begin to grow distinct. The mouth (o) is still the anus also.

Fig. 11.—Longitudinal section through a low Coelomate Worm. The primitive brain (m), or the first nerve-centre overlying the throat, has separated from the horn-plate (h). The intestinal tube has acquired a second posterior anal opening (a) in addition to the mouth-opening (a) in front, A skingland has developed into primitive kidneys (u) and opens into the body-cavity (c), which has formed between the skin-fibrous layer and the intestinal-fibrous layer.

Fig, 12.—Longitudinal section through an hypothetical Worm (Chordonium), which was among the common parent-forms of Vertebrates and Ascidians. The primitive brain (m) has lengthened into an elongated spinal tube. Between this spinal tube and the intestinal tube (d), the notochord (ch) has developed. The intestinal tube has differentiated into two divisions, an anterior gill-intestine (with three pairs of gill-openings, ks) which serves for breathing, and a posterior stomach-intestine (with a livers appendage, lb) which serves for digestion. In front, at the head-extremity, an organ of sense (q) has developed. The primitive kidney (u) opens into the body-cavity (c).

Fig. 13.—Longitudinal section through a Primitive Fish (Proselachius), closely related to the existing Sharks, and hypothetical ancestors of Man (the fins are omitted). The spinal tube has differentiated into the five primitive brain-bladders (m₁,—m₅) and the spinal marrow (m₆). (Compare Figs. 15 and 16.) The brain is enclosed in the skull (s), the spinal marrow in the vertebral canal (above the spiral marrow, the vertebral arches (wk) ; under it the vertebral bodies (wk) ; under the latter the origin of the ribs is indicated). In front an organ of sense (q, nose or eye) has developed from the horn-layer, — at the back, the primitive kidney (u). The intestinal tube (d) has differentiated into the following parts, lying one behind another: the mouth-cavity (mh), the throat cavity with six pairs of gill-openings (ks), the swimming-bladder (=lungs, lu), the oesophagus (sr), the stomach (mg), the liver (lb) with the gall-bladder (i), the small intestine (dd), and the rectum with the anus (a). Below the throat-cavity lies the heart, with the auricle (hv) and the ventricle (hk).

Fig, 14.—Longitudinal section through a human embryo of three weeks, showing the relation of the intestinal tube to its appendages. In the centre the long-stalked yelk-sac (or the navel-vesicle, ds) projects from the intestinal tube (ds); similarly the long-stalked allantois (al) projects from the intestine at the back. The heart (hz) is visible beneath the anterior intestine. Amnion-cavity (ah).

Fig. 15.—Longitudinal section through a human embryo of five weeks. (Compare Fig. 14.) The amnion and the placenta, with the urachus, are omitted. The spinal tube has differentiated into the five primitive brain-bladders (m₁,—m ₅)and the spinal marrow (m₆). (Compare Figs. 13 and 16.) The skull (s) is formed around the brain ; below the spinal marrow the series of vertebral bodies (wk). The intestinal tube has differentiated into the following divisions, lying one behind another: the throat-cavity with three pairs of gill-openings (ks), the lung (lu), the oesophagus (sr), the stomach (mg), the liver (lb), the coil of the small intestine (dd), into which the yelk-sac (ds) opens, the urinary bladder (hb), and the rectum. Heart (hs).

Fig. 16 —Longitudinal section through developed human female. All the parta are perfectly developed, but diagrammatically reduced and simplified, in order to exhibit clearly their relative positions and their relations to the four secondary germ-layers. In the brain, the five original brain-bladders (Fig. 15, m₁,—m ₅),) have been differentiated and transformed in the manner peculiar to the higher mammals: m₁,— fore brain (cerebrum), out-weighing and covering all the other four brain bladders; m₂,—twixt brain (“the centre of sight”); m₃,—, mid brain (“the four bulbs”); m₄,—, hind brain (cerebellum); m₅,—after brain, or prolonged marrow (medulla oblongata), passing into the spinal marrow (m₆,—) The brain is enclosed in the skull (s), the spinal marrow by the vertebral canal: above the spinal marrow the vertebral arches and spinal processes, under it the vertebral bodies (wk). The intestinal tube has differentiated into the following parts lying one behind another: the mouth-cavity, the throat-cavity (in which at an earlier period the gill-openings, ks, were situated), the trachea (lr) with the lungs (lu), the oesophagus (sr), the stomach (mg), the liver (1b), with the gall-bladder (i), the ventral salivary gland, or pancreas (p), the small intestine (dd), the large intestine (dc), and the rectum with the anus (a). The body-cavity, or coelom (c), is divided by the diaphragm (z) into two distinct cavities ; the breast-cavity (c), in which the heart (hz) lies in front of the lungs, and the ventral cavity in which most of the intestines lie. In front of the rectum lies the sheath (vagina, vg), which leads into the uterus (f); in this the embryo, indicated here by a small germ-membrane vesicle (e), is