Talk:1987 Developmental Stages In Human Embryos - Stage 19

Fig. 19-1. Photographs of three embryos belonging to stage 19. The trunk is now beginning to straighten, and the cervical angulation is less acute. The coalescence of parts in the pharyngeal region has altered the appearance of the auricle, and the hillocks are less conspicuous. The longitudinal axes of the upper limbs are almost at a right angle to the dorsal contour of the body of the embryo, and the axes of the upper and lower limbs are more or less parallel, so that each limb has a pre-axial and a postaxial border. The transverse and sigmoid sinuses are prominent in D. The brain in E shows, from above downward, the mesencephalon, diencephalon, and cerebral hemispheres. Top row, No. 8092. Middle row, No. 6824. Bottom row, No. 4501. All limbs are at the same magnification.

Fig. 19-2. Drawings of sections through the optic vesicle/cup and stalk at stages 13–18. As the optic vesicle is being transformed into the optic cup, the inverted (neural) lamina of the retina retains direct connection at all times with the wall of the brain. At stages 13–17 the optic ventricle communicates freely with the cavity of the diencephalon. The body of the lens is thickening and, in stage 18, the primary fibers fill the lumen of the lens.

Fig. 19-3. A continuation of the previous illustration, showing the eye and optic nerves at stages 19–23. Several sections in each embryo were combined to show the form of the optic nerve. All drawings in this and the previous figure are at the same scale.

Fig. 19-4. Drawings of the sections showing the form of the optic nerve and the progressive spread of nerve fibers from retina to brain. Some fibers have grown through the whole tract (which really is the so-called optic nerve) and are arriving at the brain at stage 20. In the more advanced stages many (neuro-ectodermal, including glial) cells are seen in longitudinal rows between bundles of nerve fibers. All drawings are at the same scale, which is slightly less than twice that of the previous two figures.

Fig. 19-5. Drawings of mid-sections of the eye at stages 19–23. The sections were selected to demonstrate the organization and relations of the various parts of the eye. The lens remains nearly spherical throughout this period and occupies a relatively large amount of the eye. The pupillary membrane is developing and the anterior chamber is emerging (stage 22). The hyaloid plexus partly fills the vitreous chamber and forms a network on the back of the lens. All drawings are at the same scale.

Fig. 19-6. Reconstructions of the cochlear duct at stages 19–23. All drawings are at the same scale. The position of the membranous labyrinth in situ has been shown by O’Rahilly and Gardner (1971, fig. 1) at stage 21 and by Streeter (1906, fig. 2) at stage 23. The relationship of the membranous labyrinth to the otic capsule at stage 23 has been illustrated by Müller and O’Rahilly (1980b, fig. 5).

Fig. 19-7. Median sections of the hypophysis cerebri at stages 19–23. Two or more sections were combined in preparing the drawings. Several sections were used to represent the lateral processes of the pars intermedia, which grow dorsally around the sides of the infundibular stem to form the pars infundibularis (or tuberalis). All drawings are at the same scale.

Fig. 19-8. The uppermost row shows (X) a general view of the brain at stage 21 (Aindicates the neurohypophysis), (Y) a left lateral view of the hypophysis cerebri, and (Z) a median section. The transverse lines A–Ein Y and Z correspond approximately to the sections shown in the middle vertical row marked 21.

The lower part of the figure consists of semidiagrammatic drawings of transverse sections of the hypophysis at stages 19- 23. One embryo was chosen for each stage, and drawings were made at five more or less evenly spaced levels of the hypophysis. The changing pattern of the various parts of the gland can be followed, stage by stage, by scanning the horizontal rows A–E. All these drawings are at the same scale.

Fig. 19-9. Drawings from the sections of the vomeronasal organ at stages 19–23. All drawings are at the same scale.

Fig. 19-10. Semidiagrammatic drawings of the submandibular gland, showing the growth of the ductal system into the mesodermal component at stages 19–23. All drawings (except the inset) are at the same scale.

Fig. 19-11. Semidiagrammatic drawings showing the origin and development of the tubules in the metanephros at stages 19–21. Practically all the embryos of stage 19 show the beginning formation of renal vesicles. Most of the specimens of stage 20 have S-shaped lumina in the vesicles, and spoon-shaped capsules have appeared in a few of the more advanced members of the group. The tubules in stage 21 show a wider range of development because new ones are being added peripherally; at the same time, those first established continue to differentiate.

Fig. 19-12. A continuation of the previous illustration, showing the further development of the metanephros at stages 22 and 23. More large glomeruli have appeared, and short secretory tubules are present in nearly all specimens. By the end of the period, tubules of the fourth or fifth generation have formed. All drawings in this and in the previous figure are at the same scale.