Template talk:Endocrine embryo table

Table Across

Organ	Stage 13	Stage 14	Stage 15	Stage 16	Stage 17	Stage 18	Stage 19	Stage 20	Stage 21	Stage 22	Stage 23
Pineal (Epiphysis)		slight irregularity in surface outline of intact head corresponds to future pineal body (O'Rahilly et al. 1982).	pineal body is detectable in roof of diencephalon (Stadium I of Turkewitsch)	cellular migration in an external direction occurs during stages 16 and 17 (Stadium 2 of Turkewitsch)		cellular migration in pineal body forms a distinct "anterior lobe" where follicles appear (Stadium 3 of Turkewitsch) (O'Rahilly 1973 a).	"anterior lobe" shows a characteristic step and wedge appearance (Stadium 4 of Turketwitsch N. Die Entwicklung der Zirbeldrüse des Menschen (The development of the pineal gland in humans). (1933) Morphol Jb 72: 379-445.)				pineal body has reached Stadium 5 of Turkewitsch)
Pituitary (Hypophysis)	basement membranes of craniopharyngeal pouch and brain are clearly in contact (O'Rahilly 1973).	craniopharyngeal pouch is prominent (Streeter 1945) and notochord appears to be inserted into its dorsal wall. The craniopharyngeal pouch has become elongated and blood vessels are beginning to grow in between the basement membranes of the pouch and brain (O'Rahilly 1973a).		slight indication of the infundibular recess may be seen in some embryos (O'Rahilly 1973 a).	juxtacerebral wall of the craniopharyngeal pouch is the thicker. The lateral lobes (future infundibular, or tuberal, part) and the anterior chamber (Vorraum) are clearly visible (O'Rahilly 1973 a). Infundibular recess displays a characteristically folded wall, namely the neurohypophysis (O'Rahilly 1973 a).		caudal part of the craniopharyngeal pouch is reduced to a closed epithelial stem.	the adenohypophysial epithelium adjacent to the neurohy- pophysis constitutes the beginning pars intermedia (O'Rahilly 1973 a). The walls of the craniopharyngeal pouch bud into the mesenchyme.	pharyngeal stalk becomes fragmented.		adenohypophysis loss of the stalk and lobules of epithelium project into the mesodermal component of the gland, and oriented epithelial follicles are present (Streeter, 1951, plate 2). Abundant angioblasts and capillaries are found.
Thyroid	median thyroid is now bilobed and is connected to the pharynx by a hollow pedicle . The telopharyngeal body has been regarded as a "lateral thyroid component" by some workers.	thyroid pedicle shows further elongation but is still connected to the epithelium of the pharynx. Right and left lobes and an isthmus may perhaps be presaged.	thyroid primordium may be detached from pharyngeal epithelium in some instances. "At about the time" when the thyroglossal duct "becomes broken it loses its lumen" (Grosser 1912).	has lost its continuity with the pharynx and it consists of two lobes, an isthmus, and a remnant of the pedicle.	lobes of the thyroid curve around the carotid arteries and are connected by a delicate isthmus. Lacunae "should not be confused with lumina of follicles".	median thyroid in contact with "lateral thyroid components" (Weller 1933) others maintain that the telopharyngeal body not be regarded as a thyroid component (Bejdl and Politzer 1953). Lobes are "composed of series of continuously communicating solid annectent bars", "the earliest stage of the definitive thyroid". First differentiation occurs in "lateral thyroid component," beginning to "blend into uniformly constituted thyroid tissue". Weller (1933) (Fig. 11) shows continuity between its pedicle and pharynx epithelium.		"annectent bars" are more compact then previously. The thyroid now exhibits its definitive external form.
Parathyroid		"Parathyrogenic zones" (Politzer G, Hann F. Uber die Emwicklung der branchiogenen Organe beim Menschen (On the development of the branchiogenic organs in humans). (1935) Z Anat Entw Gesctl 104: 671-708. are recognizable (Streeter 1945). The parathyroid 4 primordium has been illustrated at this stage by Weller (1933, Fig. 16).		parathyrogenic zones closely related to third and fourth aortic arches at 9 mm (Politzer and Hann 1935, unstaged embryo). Parathyroid 3 identifiable on anterior wall of third pharyngeal pouch (Weller 1933, Fig. 17) and "does not arise from a dorsal lobule" of pouch and "sudden appearance of well-differentiated clear chief cells in early primordia of parathyroids" at 9 mm (Norris EH. The parathyroid glands and the lateral thyroid in man: their morphogenesis, histogenesis, topographic anatomy and prenatal growth. (1937) Contrib Embryol Carneg Instn 26: 247-294).	parathyroid 4 is attached to the lateral surface of what Weller (1933) termed the "lateral thyroid component"		parathyroid 3 become detached from the pharyngeal endoderm (Jirfisek 1980).	glands are attached to lateral lobes of thyroid (Weller 1933). Weller (1933, Fig. 23) showed parathyroid 3 still rostral to parathyroid 4 at 23 mm, whereas (presumably due to variation in the "descent" of thymus) Norris (1937, Fig. 4) showed parathyroid 3 rostral to, level with, and caudal to parathyroid 4 in embryos of 16-17 mm.
Thymus‎	Weller (1933) recognized a thymic primordium "of considerable size" on ventral part of third pharyngeal pouch, Norris EH. The parathyroid glands and the lateral thyroid in man: their morphogenesis, histogenesis, topographic anatomy and prenatal growth. (1937) Contrib Embryol Carneg Instn 26: 247-294 considered this stage to be "preprimordial"	Weller's (1933) "thymus" (the third pharyngeal pouch) becomes elongated.		Norris (1938) "not until parathyroid [3] primordium has been outlined can remaining portion of third pouch be recognized, by exclusion, as the primordium of the endodermal thymus".	connection of the thymus with pharynx has been severed and thymus is intimately approximated to cervical duct (Weller 1933). According to Norris (1937), both third and fourth pouches make contact with the ectoderm, although only the third "receives an increment from the ectoderm".	thymus makes contact with thyroid gland and contains a series of canals internally (Weller 1933).		right and left components are in contact with each other (Weller 1933) but are "never completely fused" (Norris 1938, Siegler R. The thymus and the unicorn-two great myths of gross anatomy. (1969) Anat Rec. 163: 264.. Thymic cortex appears (in stages 20-22) as a result, according to Norris (1937).			cortex is well-developed, "true lobulation" has begun with appearance of" fine superficial scallops," lymphocytes are present sparsely in subcortical zone, and vessels are found within thymus (Norris 1938).
Pancreas‎	ventral pancreas may perhaps be distinguishable (Politzer 1952).	ventral pancreas (which may perhaps be distinguishable as early as stage 13) appears as an evagination from the bile duct at stages 14 (Blechschmidt 1973) and 15 (Streeter 1948). Generally described as unpaired but, at least in some cases, may perhaps be bilobed or even multiple (Delmas 1939).			ventral pancreas has now fused with dorsal (Streeter 1948). Perhaps the ventral and dorsal ducts have begun to blend (Russu and Vaida 1959).
Adrenal‎ (Suprarenal)		Cortex - A change in the characteristics of the cells of the coelomic epithelium appears between the mesogastrium and the lateral end of the mesonephros. Medulla - paravertebral sympathetic ganglia increase in size as a result of cell division and the addition of nerve fibres from the rami communicantes. The ganglia contain three types of cells: MI, M2, and M3. The M3 cells are the" parasympathetic cells" of Zuckerkandl.	Cortex - primordium is first recognizable. A new type of cell (C1) from the coelomic epithelium is found in the subjacent mesenchyme. New cells (C2) appear in the medial wall of mesonephric glomeruli and begin to migrate into the suprarenal primordium. Jirfisek (1980) denies a mesonephric contribution to the suprarenal. Medulla - all types of cells (M1, M2, and M3) increase in number. From stage 15 to stage 18, the suprarenal primordium is cigar-shaped and extends from segment T6 to segment L1, lateral to the aorta and mesogastrium.	Cortex - Another type of cell (C3) arises from the coelomic epithelium. Both C1 and C3 cells enter the suprarenal primordium. An "enormous immigration" of C2 cells occurs. Medulla - cells of neural origin are migrating into the gland, separating the cortical cells into islands. Nerve fibres from the ganglia ac- company the M1 and M3 cells. The M2 cells remain in the ganglia and become sympathetic ganglion cells.	Cortex - dorsal part of the whole suprarenal primordium is disorganized by the invasion of sympathetic nerves and cells, while the band of C2 cells and the coelomic epithelium remain intact (Crowder 1957). Medulla - first neural migration is at its height. Growth of the para-aortic complex is extensive. The plexiform complex is derived from paravertebral sympathetic ganglia T6-12 and usually L 1. Included in it are the primordia of the suprarenal medulla and of the celiac, superior mesenteric, and renal plexuses. Nerve fibres and "paraganglion" (M3) cells enter.	Cortex - gland becomes reorganized. The C1, 2, and 3 cells form cords as sinusoids develop. Cells divide at or near the surface, where new cells are added.	Cortex - C2 cells lie on gland surface and form a "capsule". Medulla - Sympathicoblasts penetrate the cortex at stages 19 and 20, and form scattered islets of medullary tissue throughout the cortex (Jirfisek 1980).		Cortex - cellular "capsule" becomes covered by a layer of fibrous tissue.	Cortex - C2 cells have changed and resemble fibrocytes.

Table Down

Week	Carnegie Stage	Pineal (Epiphysis)	Pituitary (Hypophysis)	Thyroid	Parathyroid	Thymus	Pancreas	Adrenal (Suprarenal) Cortex/Medulla
Week 4	Stage 13		basement membranes of the craniopharyngeal pouch and the brain are clearly in contact (O'Rahilly 1973).	median thyroid is now bilobed and is connected to the pharynx by a hollow pedicle (Weller 1933). The telopharyngeal body has been regarded as a "lateral thyroid component" by some workers (e.g. Weller 1933).		Weller (1933) recognized already a thymic primordium "of considerable size" on the ventral part of the third pharyngeal pouch, whereas Norris (1938) considered this stage to be "preprimordial"	ventral pancreas may perhaps be distinguishable (Politzer 1952).
Week 5	stage 14	a slight irregularity in the surface outline of the intact head corresponds to the future pineal body (O'Rahilly et al. 1982). Thymus. Weller's (1933) "thymus" (the third pharyngeal pouch) becomes elongated.	craniopharyngeal pouch is prominent (Streeter 1945) and the notochord appears to be inserted into its dorsal wall. The craniopharyngeal pouch has become elongated and blood vessels are beginning to grow in between the basement membranes of the pouch and brain (O'Rahilly 1973a).	thyroid pedicle shows further elongation but is still connected to the epithelium of the pharynx (Weller 1933). Right and left lobes and an isthmus may perhaps be presaged (ibid.).	"Parathyrogenic zones" (Politzer and Hann 1935) are recognizable (Streeter 1945). The parathyroid 4 primordium has been illustrated at this stage by Weller (1933, Fig. 16).		ventral pancreas (which may perhaps be distinguishable as early as stage 13) appears as an evagination from the bile duct at stages 14 (Blechschmidt 1973) and 15 (Streeter 1948). It is generally described as unpaired but, at least in some cases, may perhaps be bilobed or even multiple (Delmas 1939).	Cortex - A change in the characteristics of the cells of the coelomic epithelium appears between the mesogastrium and the lateral end of the mesonephros. Medulla - paravertebral sympathetic ganglia increase in size as a result of cell division and the addition of nerve fibres from the rami communicantes. The ganglia contain three types of cells: MI, M2, and M3. The M3 cells are the" parasympathetic cells" of Zuckerkandl.
Week 5	stage 15	pineal body is detectable in the roof of the diencephalon (Stadium I of Turkewitsch 1933) (O'Rahilly 1968).		thyroid primordium may be detached from the pharyngeal epithelium in some instances. "At about the time" when the thyroglossal duct "becomes broken it loses its lumen" (Grosser 1912).				Cortex - primordium is first recognizable. A new type of cell (C1) from the coelomic epithelium is found in the subjacent mesenchyme. New cells (C2) appear in the medial wall of mesonephric glomeruli and begin to migrate into the suprarenal primordium. Jirfisek (1980) denies a mesonephric contribution to the suprarenal. Medulla - all types of cells (M1, M2, and M3) increase in number. From stage 15 to stage 18, the suprarenal primordium is cigar-shaped and extends from segment T6 to segment L1, lateral to the aorta and mesogastrium.
Week 6	stage 16	cellular migration in an external direction occurs in the pineal body during stages 16 and 17 (Stadium 2 of Turkewitsch 1933) (O'Rahilly 1968).	slight indication of the infundibular recess may be seen in some embryos (O'Rahilly 1973 a).	has lost its continuity with the pharynx and it consists of two lobes, an isthmus, and a remnant of the pedicle (Weller 1933).	parathyrogenic zones are closely related to the third and fourth aortic arches at 9 mm (Politzer and Hann 1935, unstaged embryo). Parathyroid 3 is identifiable on the anterior wall of the third pharyngeal pouch (Weller 1933, Fig. 17) and "does not arise from a dorsal lobule" of the pouch (Norris 1937). The "sudden appearance of well-differentiated clear chief cells in the early primordia of the parathyroids" at 9 mm was emphasized by Norris (1937).	according to Norris (1938), "not until the primordium of the parathyroid [3] has been outlined can the remaining portion of the third pouch be recognized, by exclusion, as the primordium of the endodermal thymus".	dorsal pancreas and the ventral pancreas are contiguous.	Cortex - Another type of cell (C3) arises from the coelomic epithe- lium. Both C1 and C3 cells enter the suprarenal primordium. An "enormous immigration" of C2 cells occurs. Medulla - cells of neural origin are migrating into the gland, separating the cortical cells into islands. Nerve fibres from the ganglia ac- company the M1 and M3 cells. The M2 cells remain in the ganglia and become sympathetic ganglion cells.
Week 6	stage 17		juxtacerebral wall of the craniopharyngeal pouch is the thicker. The lateral lobes (future infundibular, or tuberal, part) and the anterior chamber (Vorraum) are clearly visible (O'Rahilly 1973 a). The infundibular recess displays a characteristically folded wall, namely the neurohypophysis (O'Rahilly 1973 a).	lobes of the thyroid curve around the carotid arteries and are connected by a delicate isthmus. Lacunae "should not be confused with lumina of follicles" (Weller 1933).	parathyroid 4 is attached to the lateral surface of what Weller (1933) termed the "lateral thyroid component"	connection of the thymus with the pharynx has been severed (Weller 1933). The thymus is intimately approximated to the cervical duct (ibid.) According to Norris (1937), both third and fourth pouches make contact with the ectoderm, although only the third "receives an increment from the ectoderm".	ventral pancreas has now fused with dorsal (Streeter 1948). Perhaps the ventral and dorsal ducts have begun to blend (Russu and Vaida 1959).	Cortex - dorsal part of the whole suprarenal primordium is disorganized by the invasion of sympathetic nerves and cells, while the band of C2 cells and the coelomic epithelium remain intact (Crowder 1957). Medulla - first neural migration is at its height. Growth of the para-aortic complex is extensive. The plexiform complex is derived from paravertebral sympathetic ganglia T6-12 and usually L 1. Included in it are the primordia of the suprarenal medulla and of the celiac, superior mesenteric, and renal plexuses. Nerve fibres and "paraganglion" (M3) cells enter.
Week 7	stage 18	cellular migration in the pineal body forms a distinct "anterior lobe" in which follicles appear (Stadium 3 of Turkewitsch 1933) (O'Rahilly 1973 a).	median thyroid is in contact with "lateral thyroid components" (Weller 1933) but others have maintained that the telopharyngeal body should not be regarded as a thyroid component (Bejdl and Politzer 1953). The lobes of the thyroid are "composed of series of continuously communicating solid annectent bars" (Weller 1933). This is "the earliest stage of the definitive thyroid" (ibid.). First differentiation occurs in Weller's (1933) "lateral thyroid component," which is beginning to "blend into uniformly constituted thyroid tissue". Weller (1933) illustrated (Fig. 11) a thyroid gland that still showed continuity between its pedicle and the epithelium of the pharynx.			thymus makes contact with the thyroid gland and contains a series of canals internally (Weller 1933).		Cortex - gland becomes reorganized. The C1, 2, and 3 cells form cords as sinusoids develop. Cells divide at or near the surface, where new cells are added.
Week 7	stage 19	"anterior lobe" of the pineal body shows a characteristic step and wedge appearance (Stadium 4 of Turkewitsch 1933) (O'Rahilly 1968).	caudal part of the craniopharyngeal pouch is reduced to a closed epithelial stem (Andersen et al. 1971).		parathyroid 3 become detached from the pharyngeal endoderm (Jirfisek 1980).			Cortex - C2 cells lie on the surface of the gland and form a "capsule". Medulla - Sympathicoblasts penetrate the cortex at stages 19 and 20, and form scattered islets of medullary tissue throughout the cortex (Jirfisek 1980).
Week 8	stage 20		adenohypophysial epithelium adjacent to the neurohy- pophysis constitutes the beginning pars intermedia (O'Rahilly 1973 a). The walls of the craniopharyngeal pouch bud into the mesenchyme (An- dersen et al. 1971 ; Jir/tsek 1980).	"annectent bars" of the thyroid are more compact then previously (Weller 1933). The thyroid now exhibits its definitive external form.	parathyroid glands are attached to the lateral lobes of the thyroid (Weller 1933). Weller (1933, Fig. 23) showed parathyroid 3 still rostral to parathyroid 4 at 23 mm, whereas (presumably due to variation in the "descent" of the thymus) Norris (1937, Fig. 4) showed parathyroid 3 rostral to, level with, and caudal to parathyroid 4 in embryos of 16-17 mm.	right and left components are in contact with each other (Weller 1933) but are "never completely fused" (Norris 1938, Siegler 1969). Thymic cortex appears (in stages 20-22) as a result, according to Norris (1938), of migration of and covering by "cells derived from the cervical sinus".
Week 8	stage 21		pharyngeal stalk becomes fragmented (Jirfisek 1980).					Cortex - the cellular "capsule" becomes covered by a layer of fibrous tissue.
Week 8	stage 22				Parathyroids 4 become detached from the pharyngeal endoderm (Jirfisek 1980).			Cortex - the C2 cells have changed and resemble fibrocytes.
Week 8	stage 23	pineal body has reached Stadium 5 of Turkewitsch (1933).	adenohypophysis loss of the stalk and lobules of epithelium project into the mesodermal component of the gland, and oriented epithelial follicles are present (Streeter, 1951, plate 2). Abundant angioblasts and capillaries are found.			cortex is well-developed, "true lobulation" has begun with the appearance of" fine superficial scallops," lymphocytes are present sparsely in the subcortical zone, and vessels are found within the thymus (Norris 1938).		Cortex - It appears that C2 cells first enter the body of the gland at this stage. The pattern of the arterial supply is established. The cellular "capsule" is penetrated by arterial capillaries which join the sinusoids. Their points of entry give the surface of the gland an appearance of cobblestones. The zona glomerulosa is formed of CI and C3 cells. Cells from this zone and from the "capsule" migrate centrally into the cords. Medulla - Nerve fibres and neuroblasts are first seen in the body of the gland. The paragangtion (M3) cells are beginning to multiply rapidly and, from 30 mm (stage 23) until birth, some are differentiating into chromaffin cells.

Pineal

Week	Carnegie Stage	Pineal (Epiphysis)
Week 4	13
Week 5	14	a slight irregularity in the surface outline of the intact head corresponds to the future pineal body (O'Rahilly et al. 1982).
	15	pineal body is detectable in the roof of the diencephalon (Stadium I of Turkewitsch 1933) (O'Rahilly 1968).
Week 6	16	cellular migration in an external direction occurs in the pineal body during stages 16 and 17 (Stadium 2 of Turkewitsch 1933) (O'Rahilly 1968).
	17
Week 7	18	cellular migration in the pineal body forms a distinct "anterior lobe" in which follicles appear (Stadium 3 of Turkewitsch 1933) (O'Rahilly 1973 a).
	19	"anterior lobe" of the pineal body shows a characteristic step and wedge appearance (Stadium 4 of Turkewitsch 1933) (O'Rahilly 1968).
Week 8	20
	21
	22
	23	pineal body has reached Stadium 5 of Turkewitsch (1933).

Human Endocrine - Embryonic

Based on data from O'Rahilly R. The timing and sequence of events in the development of the human endocrine system during the embryonic period proper. (1983) Anat. Embryol., 166: 439-451. PMID 6869855

By Stage

Pineal | Hypothalamus‎ | Pituitary | Thyroid | Parathyroid | Thymus‎ | Pancreas‎ | Adrenal‎ | Gonad‎ |

Stage 13

• Hypophysis - basement membranes of the craniopharyngeal pouch and the brain are clearly in contact (O'Rahilly 1973).
• Thymus - Weller (1933) recognized already a thymic primordium "of considerable size" on the ventral part of the third pharyngeal pouch, whereas Norris (1938) considered this stage to be "preprimordial"
• Thyroid - median thyroid is now bilobed and is connected to the pharynx by a hollow pedicle (Weller 1933). The telopharyngeal body has been regarded as a "lateral thyroid component" by some workers (e.g. Weller 1933).
• Pancreas - ventral pancreas may perhaps be distinguishable (Politzer 1952).

Stage 14

• Hypophysis - craniopharyngeal pouch is prominent (Streeter 1945) and the notochord appears to be inserted into its dorsal wall. The craniopharyngeal pouch has become elongated and blood vessels are beginning to grow in between the basement membranes of the pouch and brain (O'Rahilly 1973a).
• Epiphysis - a slight irregularity in the surface outline of the intact head corresponds to the future pineal body (O'Rahilly et al. 1982). Thymus. Weller's (1933) "thymus" (the third pharyngeal pouch) becomes elongated.
• Parathyroids - "Parathyrogenic zones" (Politzer and Hann 1935) are recognizable (Streeter 1945). The parathyroid 4 primordium has been illustrated at this stage by Weller (1933, Fig. 16).
• Thyroid - thyroid pedicle shows further elongation but is still connected to the epithelium of the pharynx (Weller 1933). Right and left lobes and an isthmus may perhaps be presaged (ibid.).
• Adrenal Cortex - A change in the characteristics of the cells of the coelomic epithelium appears between the mesogastrium and the lateral end of the mesonephros.
• Adrenal Medulla - paravertebral sympathetic ganglia increase in size as a result of cell division and the addition of nerve fibres from the rami communicantes. The ganglia contain three types of cells: MI, M2, and M3. The M3 cells are the" parasympathetic cells" of Zuckerkandl.
• Pancreas - ventral pancreas (which may perhaps be distinguishable as early as stage 13) appears as an evagination from the bile duct at stages 14 (Blechschmidt 1973) and 15 (Streeter 1948). It is generally described as unpaired but, at least in some cases, may perhaps be bilobed or even multiple (Delmas 1939).

Stage 15

• Epiphysis - pineal body is detectable in the roof of the diencephalon (Stadium I of Turkewitsch 1933) (O'Rahilly 1968).
• Thyroid - thyroid primordium may be detached from the pharyngeal epithelium in some instances. "At about the time" when the thyroglossal duct "becomes broken it loses its lumen" (Grosser 1912).
• Adrenal Cortex - primordium is first recognizable. A new type of cell (C1) from the coelomic epithelium is found in the subjacent mesenchyme. New cells (C2) appear in the medial wall of mesonephric glomeruli and begin to migrate into the suprarenal primordium. Jirfisek (1980) denies a mesonephric contribution to the suprarenal.
• Adrenal Medulla - all types of cells (M1, M2, and M3) increase in number. From stage 15 to stage 18, the suprarenal primordium is cigar-shaped and extends from segment T6 to segment L1, lateral to the aorta and mesogastrium.

Stage 16

• Hypophysis - slight indication of the infundibular recess may be seen in some embryos (O'Rahilly 1973 a).
• Epiphysis - cellular migration in an external direction occurs in the pineal body during stages 16 and 17 (Stadium 2 of Turkewitsch 1933) (O'Rahilly 1968).
• Thymus - according to Norris (1938), "not until the primordium of the parathyroid [3] has been outlined can the remaining portion of the third pouch be recognized, by exclusion, as the primordium of the endodermal thymus".
• Parathyroids - parathyrogenic zones are closely related to the third and fourth aortic arches at 9 mm (Politzer and Hann 1935, unstaged embryo). Parathyroid 3 is identifiable on the anterior wall of the third pharyngeal pouch (Weller 1933, Fig. 17) and "does not arise from a dorsal lobule" of the pouch (Norris 1937). The "sudden appearance of well-differentiated clear chief cells in the early primordia of the parathyroids" at 9 mm was emphasized by Norris (1937).
• Thyroid - has lost its continuity with the pharynx and it consists of two lobes, an isthmus, and a remnant of the pedicle (Weller 1933).
• Adrenal Cortex. Another type of cell (C3) arises from the coelomic epithe- lium. Both C1 and C3 cells enter the suprarenal primordium. An "enormous immigration" of C2 cells occurs.
• Adrenal Medulla - cells of neural origin are migrating into the gland, separating the cortical cells into islands. Nerve fibres from the ganglia ac- company the M1 and M3 cells. The M2 cells remain in the ganglia and become sympathetic ganglion cells.
• Pancreas - dorsal pancreas and the ventral pancreas are contiguous.

Stage 17

• Hypophysis - juxtacerebral wall of the craniopharyngeal pouch is the thicker. The lateral lobes (future infundibular, or tuberal, part) and the anterior chamber (Vorraum) are clearly visible (O'Rahilly 1973 a). The infundibular recess displays a characteristically folded wall, namely the neurohypophysis (O'Rahilly 1973 a).
• Thymus - connection of the thymus with the pharynx has been severed (Weller 1933). The thymus is intimately approximated to the cervical duct (ibid.) According to Norris (1937), both third and fourth pouches make contact with the ectoderm, although only the third "receives an increment from the ectoderm".
• Parathyroids -parathyroid 4 is attached to the lateral surface of what Weller (1933) termed the "lateral thyroid component"
• Thyroid. The lobes of the thyroid curve around the carotid arteries and are connected by a delicate isthmus. Lacunae "should not be confused with lumina of follicles" (Weller 1933).
• Adrenal Cortex - dorsal part of the whole suprarenal primordium is disorganized by the invasion of sympathetic nerves and cells, while the band of C2 cells and the coelomic epithelium remain intact (Crowder 1957).
• Adrenal Medulla - first neural migration is at its height. Growth of the para-aortic complex is extensive. The plexiform complex is derived from paravertebral sympathetic ganglia T6-12 and usually L 1. Included in it are the primordia of the suprarenal medulla and of the celiac, superior mesenteric, and renal plexuses. Nerve fibres and "paraganglion" (M3) cells enter.
• Pancreas - ventral pancreas has now fused with dorsal (Streeter 1948). Perhaps the ventral and dorsal ducts have begun to blend (Russu and Vaida 1959).

Stage 18

• Epiphysis - cellular migration in the pineal body forms a distinct "anterior lobe" in which follicles appear (Stadium 3 of Turkewitsch 1933) (O'Rahilly 1973 a).
• Thymus- thymus makes contact with the thyroid gland and contains a series of canals internally (Weller 1933).
• Thyroid - median thyroid is in contact with "lateral thyroid compo- nents" (Weller 1933) but others have maintained that the telopharyngeal body should not be regarded as a thyroid component (Bejdl and Politzer 1953). The lobes of the thyroid are "composed of series of continuously communicating solid annectent bars" (Weller 1933). This is "the earliest stage of the definitive thyroid" (ibid.). First differentiation occurs in Weller's (1933) "lateral thyroid component," which is beginning to "blend into uniformly constituted thyroid tissue". Weller (1933) illustrated (Fig. 11) a thyroid gland that still showed continuity between its pedicle and the epithelium of the pharynx.
• Adrenal Cortex - gland becomes reorganized. The C1, 2, and 3 cells form cords as sinusoids develop. Cells divide at or near the surface, where new cells are added.

Stage 19

• Hypophysis - the caudal part of the craniopharyngeal pouch is reduced to a closed epithelial stem (Andersen et al. 1971).
• Epiphysis - the "anterior lobe" of the pineal body shows a characteristic step and wedge appearance (Stadium 4 of Turkewitsch 1933) (O'Rahilly 1968).
• Parathyroids - 3 become detached from the pharyngeal endo- derm (Jirfisek 1980).
• Adrenal Cortex - C2 cells lie on the surface of the gland and form a "capsule".
• Adrenal Medulla - Sympathicoblasts penetrate the cortex at stages 19 and 20, and form scattered islets of medullary tissue throughout the cortex (Jirfisek 1980).

Stage 20

• Hypophysis - the adenohypophysial epithelium adjacent to the neurohy- pophysis constitutes the beginning pars intermedia (O'Rahilly 1973 a).

The walls of the craniopharyngeal pouch bud into the mesenchyme (An- dersen et al. 1971 ; Jir/tsek 1980).

• Thymus - the right and left components are in contact with each other (Weller 1933)but are "never completely fused" (Norris 1938, Siegler 1969). Thymic cortex appears (in stages 20-22) as a result, according to Norris

(1938), of migration of and covering by "cells derived from the cervical sinus".

• Parathyroids - the parathyroid glands are attached to the lateral lobes of the thyroid (Weller 1933).

Weller (1933, Fig. 23) showed parathyroid 3 still rostral to parathyroid 4 at 23 mm, whereas (presumably due to variation in the "descent" of the thymus) Norris (1937, Fig. 4) showed parathyroid 3 rostral to, level with, and caudal to parathyroid 4 in embryos of 16-17 mm.

• Thyroid - the "annectent bars" of the thyroid are more compact then previously (Weller 1933). The thyroid now exhibits its definitive external form.

Stage 21

• Hypophysis - the pharyngeal stalk becomes fragmented (Jirfisek 1980).
• Adrenal Cortex - the cellular "capsule" becomes covered by a layer of fibrous tissue.

Stage 22

• Parathyroids - Parathyroids 4 become detached from the pharyngeal endoderm (Jirfisek 1980).
• Adrenal Cortex - the C2 cells have changed and resemble fibrocytes.

Stage 23

• Pituitary - adenohypophysis loss of the stalk and lobules of epithelium project into the mesodermal component of the gland, and oriented epithelial follicles are present (Streeter, 1951, plate 2). Abundant angioblasts and capillaries are found.
• Epiphysis - The pineal body has reached Stadium 5 of Turkewitsch (1933).
• Thymus - The cortex is well-developed, "true lobulation" has begun with the appearance of" fine superficial scallops," lymphocytes are present sparsely in the subcortical zone, and vessels are found within the thymus (Norris 1938).
• Adrenal Cortex - It appears that C2 cells first enter the body of the gland at this stage. The pattern of the arterial supply is established. The cellular "capsule" is penetrated by arterial capillaries which join the sinusoids. Their points of entry give the surface of the gland an appearance of cobblestones. The zona glomerulosa is formed of CI and C3 cells. Cells from this zone and from the "capsule" migrate centrally into the cords.
• Adrenal Medulla - Nerve fibres and neuroblasts are first seen in the body of the gland. The paragangtion (M3) cells are beginning to multiply rapidly and, from 30 mm (stage 23) until birth, some are differentiating into chromaffin cells.