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The "Standard Event System" (SES) to Study Vertebrate Embryos was developed in 2009 to establish a common language in comparative embryology. Homologous developmental characters are defined therein and should be recognisable in all vertebrate embryos. The SES includes a protocol on how to describe and depict vertebrate embryonic characters. The SES was initially developed for external developmental characters of organogenesis, particularly for turtle embryos. However, it is expandable both taxonomically and in regard to anatomical or molecular characters. This article should act as an overview on the species staged with SES and document the expansions of this system. New entries need to be validated based on the citation of scientific publications. The guideline on how to establish new SES-characters and to describe species can be found in the original paper of Werneburg (2009).

SES-characters are used to reconstruct ancestral developmental sequences in evolution such as that of the last common ancestor of placental mammals. Also the plasticity of developmental characters can be documented and analysed.

SES-staged species

Overview on the vertebrate species staged with SES.

SES-characters

New SES-characters are continuously described in new publications. Currently, characters of organogenesis are described for Vertebrata (V), Gnathostomata (G), Tetrapoda (T), Amniota (A), Sauropsida (S), Squamata (SQ), Mammalia (M), and Monotremata (MO). In total, 166 SES-characters are currently defined.

Character complex	Character	Character description	Reference	Illustration
A. Egg (V01).	A1. Egg lay (V01a).	Most authors begin to create their staging tables shortly after or around egg lay time.
B. Blastula (V02).	B1. Blastoporus (V02a).	The blastoporus begins to form as a lip and is later on visible as a clear depression in the blastula.
C. Neural tube (V03).	C1. Primitive streak present (V03a).	The neural plate starts folding. The lateral primitive folds border the primitive groove.
C. Neural tube (V03).	C2. Neural folds begin to close (V03b).	The dorsal borders of the neural folds come in touch and begin to form the neural tube that encloses the neural tube. The anterior and posterior regions of the primitive streak remain open.
C. Neural tube (V03).	C3. Anterior neuropore closed (V03c).	The neural tube closes anteriorly by final caudal fusion of the primitive folds.
C. Neural tube (V03).	C4. Posterior neuropore closed (V03d).	The neural tube closes posteriorly by final caudal fusion of the primitive folds.
D. Somites.	D1. Somites hard to count (V04a).	Correlated to the internal differentiation and partly due to the carapace forming in turtles somite borders become blurred in specific regions until somites are completely inconspicuous.
D. Somites.	D2. 1-5 somite pairs (V04b).	The total number of somite pairs is count, filled in a formula for each specimen and afterwards grouped within the somite cluster of five somites each. Seldom the left and the right side show a different number of mesodermal segments. By definition the row with the maximum is to be count. Often a somite pair underlies a forming process. This one should also be counted.
D. Somites.	D3. 6-10 somite pairs (V04c).	compare to character D2		see D2
D. Somites.	D4. 11-15 somite pairs (V04d).	compare to character D2		see D2
D. Somites.	D5. 16-20 somite pairs (V04e).	compare to character D2		see D2
D. Somites.	D6. 21-25 somite pairs (V04f).	compare to character D2		see D2
D. Somites.	D7. 26-30 somite pairs (V04g).	compare to character D2		see D2
D. Somites.	D8. 31-35 somite pairs (V04h).	compare to character D2		see D2
D. Somites.	D9. 36-40 somite pairs (V04i).	compare to character D2		see D2
D. Somites.	D10. 41-45 somite pairs (V04j).	compare to character D2		see D2
D. Somites.	D12. 51-55 somite pairs (V04l).	compare to character D2		see D2
D. Somites.	D13. 56-60 somite pairs (V04m).	compare to character D2		see D2
D. Somites.	D14. 61- somite pairs (V04n).	Sixty one or more somite pairs, character list to be continued if needed.		see D2
E. Head (V05)	E1. Head bulbus (V05a)	The head is formed as a distinct "bulbus" and a slight strangling (neck) restricts the head region from the thoracal region.
E. Head (V05)	E2. Anterior cephalic projection (V05b).	The neck (pharyngeal area) elongates and is at least as long as broad.
E. Head (V05)	E3. Head projection disappeared (V05c).	The forming of a mesencephalic (or/and a diencephalic) projection in the posterodorsal region of the head is characterized by a continuing growth and can not be defined as a distinguishable event. But the disappearance of the structure resulting in a flat occipital head region can be well defined and is possibly associated to skull ossification.
F. Nose (V06)	F1. Olfactory placode (V06a).	The nasal placode occurs.
F. Nose (V06)	F2. External nares (V06b).	Deep furrows between the frontonasal and maxillary processes develop and are visible as invaginations in the nasal region.
F. Nose (V06)	F3. Nostrils formed (V06c).	The external nares are closed and nostrils are formed.
G. Ear (V07).	G1. Otic placode (V07a).	The otic placode occurs.
G. Ear (V07).	G2. Otic vesicle (V07b).	The ear is formed by an invagination and a capsule is forming that becomes large in turtles.
G. Ear (V07).	G3. Otic capsule inconspicuous (V07c).	The skin becomes intransparent and the otic capsule is no longer visible.
G. Ear (V07).	G4. Pinna fold (V07d).	A pinna fold is formed in mammals.
G. Ear (V07).	G5. Tragus (V07e).	N.N.		N.N.
H. Eye (V08).	H1. Optic vesicle (V08a).	An optic vesicle forms lateral of the prosencephalic region. It can be mistaken for the trigeminal ganglion which is proportional enlarged in Tachyglossus or Monodelphis at this early period of development. But the ganglion lies more caudally.
H. Eye (V08).	H2. Lens placode (V08b).	The lens placode forms in the middle of the optic cup and primarily has only an indistinct contour.
H. Eye (V08).	H3. Optic fissure (V08c).	The optic or choroid fissure represents the blood vessel agglomeration, which supplies the developing lens. It forms a clear streak between the lens and the ventral most curvature of the optic cup. When the lens is completely formed the optic fissure disappears, which is a fluent process that can not be defined as a distinct event.
H. Eye (V08).	H4. Contour lens/iris (V08d).	The lens is completely formed and shows a distinct contour. Parallel the iris gets its contour but is often hard to distinguish from the lens.		see H3
H. Eye (V08).	H5. Pupil forms (V08e).	In the middle of the optic cup/lens region the pupil forms. That reflects the complete formation of the iris musculature.
H. Eye (V08).	H6. Scleral papillae (V08f).	Between the lens and the border of the optic cup circularly scleral bones occur.
H. Eye (V08).	H7. Scleral papillae inconspicuous (V08g).	The scleral papillae become inconspicuous.		see H6
I. Ribs (V09).	I1. Rib primordial visible (V09a).	Rib primordia are visible through the carapace or skin for a short time of development.
J. Heart (V10).	J1. Ventricle bulbus (V10a).	Description The developing heart (ventricle) primordium forms a small bulging in the ventral thoracal region.
J. Heart (V10).	J2. Thoracal bulbus disappeared (V10b).	During development the ventricle and the liver form a common thoracal bulbus which disappears when the plastron forms in turtles or ribs ossify.
J. Heart (V10).	J3. Ventricle S-shaped (V10c).	The ventricle forms a curvature that gives the ventricle an S-shaped look.
K. Tail (V11).	K1. Tail bud (V11a).	Although in many references a tail is arbitrarily described very early or in association to the development to the hind limb bud here the occurrence of the tail bud is defined as a distinct constriction of the caudal body region.
L. Limbs (V12).	L1. Forelimb ridge (V12a).	It is a lateral eruption in the anterior part of body wall that is wider (anterior-posterior) than long (proximal-distal).
L. Limbs (V12).	L2. Hind limb ridge (V12o).	It is a lateral eruption in the posterior part of body wall that is wider (anterior-posterior) than long (proximal-distal).		see L1.
L. Limbs (V12).	L3. Forelimb bud (V12b).	The forelimb bud is about as broad as wide.
L. Limbs (V12).	L4. Hind limb bud (V12s).	The hind limb bud is about as broad as wide.		see L3
L. Limbs (V12).	L5. Forelimb elongated (V12c).	The forelimb bud is longer (proximal-distal) than wide (anterior-posterior).
L. Limbs (V12).	L6. Hind limb elongated (V12p).	The hind limb bud is longer (proximal-distal) than wide (anterior-posterior).		see L5
L. Limbs (V12).	L7. Forelimb AER (V12d).	On the distal part of the forelimb bud an apical ectodermal ridge (AER) is formed in a horizontal longitude. Often it is only visible as a slightly eruption and in a particular angle of view.
L. Limbs (V12).	L8. Hindlimb AER (V12e).	On the distal part of the hindlimb bud an apical ectodermal ridge (AER) is formed in a horizontal longitude.		see L7
L. Limbs (V12).	L9. Forelimb elbow (V12f).	The forelimb develops an elbow.
L. Limbs (V12).	L10. Hind limb knee (V12t).	The hind limb develops an knee.		see L9
L. Limbs (V12).	L11. Forelimb paddle (V12g).	The forelimb is thicker distally than proximally. Paddle shaped.
L. Limbs (V12).	L12. Hind limb paddle (V12h).	The hind limb is thicker distally than proximally. Paddle shaped.		see L11
L. Limbs (V12).	L13. Forelimb digital plate (V12i).	In the distal region of the forelimb a round digital plate is formed by flattening of its paddle like end in a horizontal plane. The digital plate is clearly separated from the tube shaped leg by a surrounding step.
L. Limbs (V12).	L14. Hindlimb digital plate (V12j).	In the distal region of the hindlimb a digital plate forms.		see L13
L. Limbs (V12).	L15. Digital grooves forelimb (V12k).	On the forelimb digital grooves/ridges are visible. Sometimes grooves develop simultaneously, sometimes only one or two ridges are visible first.
L. Limbs (V12).	L16. Digital grooves hind limb (V12u).	On the hind limb digital grooves/ridges are visible. Sometimes grooves develop simultaneously, sometimes only one or two ridges are visible first.		see L15
L. Limbs (V12).	L17. Digital serration forelimb (V12l).	The periphery of the digital plate is slightly serrated on the forelimb
L. Limbs (V12).	L18. Digital serration hind limb (V12v).	The periphery of the digital plate is slightly serrated on the hind limb.		see L17
L. Limbs (V12).	L19. Finger (V12m).	At least one forelimb phalanx projects beyond the digital web and is longer than wide.		N.N.
L. Limbs (V12).	L20. Toe (V12w).	At least one hind limb phalanx projects beyond the digital web and is longer than wide.
L. Limbs (V12).	L21. First claw forelimb (V12n).	At least one claw develops on the forelimb.
L. Limbs (V12).	L22. First claw hind limb (V12x).	At least one claw develops on the hind limb.		see L21
L. Limbs (V12).	L23. Hind limb sporn (V12q).	On the medial face of the hindlimb the poison sporn of Monotremata is visible.
L. Limbs (V12).	L24. Hind limb sporn pouch (V12r).	On the medial face of the hindlimb a skin fold is formed where the poison sporn of Monotremata can be retracted.		N.N.
L. Limbs (V12).	L25. Membranes between the fingers are completely gone (V12s).	see character name
L. Limbs (V12).	L26. Membranes between the toes are completely gone (V12t).	see character name		see L25
L. Limbs (V12).	L27. Separate phalanges are visible in the fingers (V12u).	see character name
L. Limbs (V12).	L28. Separate phalanges are visible in the toes (V12v).	see character name		see L27
M. Scales, etc. (V13).	M1. Head scales (V13a).	Scales on the dorsum of the head occur. The forming of scutes on throat and lower eyelid are encoded separately. Due feathers and scales are assumed to be homologous the scale characters are also applicable to bird development. Mammalian hears are formed differently and are not regarded here.
M. Scales, etc. (V13).	M2. Throat scales (V13b).	Scales between the lower jaw and the ventral neck region occur.
M. Scales, etc. (V13).	M3. Lower eyelid scales (V13c).	Scales on the lower eyelid occur.
M. Scales, etc. (V13).	M4. Neck scales (V13d).	Scales on the neck occur. This character is sometimes difficult to define in cryptodire turtles due to the head/neck retraction.
M. Scales, etc. (V13).	M5. Back scales (V13e).	Scales on the back (the dorsal) of the trunk. Scales on the carapace in turtles are encoded separately.
M. Scales, etc. (V13).	M6. Forelimb scales proximal (V13f).	The first occurrence of scales on a limb. Generally on the upper part of the forelimb.
M. Scales, etc. (V13).	M7. Forelimb scales distal (V13g).	The forelimb is completely covered by scales, which includes the cover of the whole leg and the digital region.		see M6
M. Scales, etc. (V13).	M8. Hind limb scales proximal (V13h).	The first occurrence of scales on a limb. Generally on the upper part of the forelimb.
M. Scales, etc. (V13).	M9. Hind limb scales distal (V13i).	The forelimb is completely covered by scales, which includes the cover of the whole leg and the digital region.		see M8
M. Scales, etc. (V13).	M10. Tail scales (V13j).	Scales on the tail occur.
M. Scales, etc. (V13).	M11. Carapace scales (V13k).	Horny scutes on the turtle carapace develop already in the egg. They are difficult to demonstrate in photographs.
N. Hatch / birth / leaving pouch / weaning (V14).	N1. Hatch (V14a).	Mostly, but not necessarily the hatch represents the end of the embryonic, organ differentiating period in egg laying vertebrates.
N. Hatch / birth / leaving pouch / weaning (V14).	N2. Birth (V14b).	The fetus is born.
N. Hatch / birth / leaving pouch / weaning (V14).	N3. Leaving pouch (V14c).	In marsupials, the young are leaving the pouch for the first time.
N. Hatch / birth / leaving pouch / weaning (V14).	N4. Weaning (V14d).	End of lactation.
O. Maxillary process of the mandibular arch (G01).	O1. Maxillary bud (G01a).	The maxillary process of the mandibular arch occurs as a bud posterior to the eye. Often it is clearly seen as the anterodorsal process of the first pharyngeal arch.		|-
O. Maxillary process of the mandibular arch (G01).	O2. Maxillary process posterior to eye (G01b).	The maxillary process lies posterior to the eye for a long period. First, when a clear rostrad development of the maxillary process is recognizable, its position at the level of the posterior margin of the optic cup should be noted. For identifying the level of the maxillary process in respect to the eye the ventral border of the telencephalic/diencephalic head region must be orientated horizontally.
O. Maxillary process of the mandibular arch (G01).	O3. Maxillary process midline of eye (G01c).	The tip of the maxillary process is located at the level of the optic (choroid) fissure, around the midline of the eye.
O. Maxillary process of the mandibular arch (G01).	O4. Maxillary process anterior to lens (G01d).	The tip of the maxillary process is located beyond the optic fissure and is situated around the level of the anterior borders of pupil, iris, lens and scleral papillae.
O. Maxillary process of the mandibular arch (G01).	O5. Maxillary process anterior to eye (G01e).	The tip of the maxillary process is located beyond the level of the anterior border of the eye up to the posterior margin of frontonasal process.
O. Maxillary process of the mandibular arch (G01).	O6. Maxillary process fuses with frontonasal process (G01f).	The maxillary process fuses with the frontonasal process and both form a more or less consistent upper jaw.
P. Mandibular process (G02).	P1. Mandibular arch bud (G02a).	The first pharyngeal (mandibular) arch is generally the first pharyngeal arch to occur as a bud. It forms later on a dorsal maxillary and a ventral mandibular process.
P. Mandibular process (G02).	P2. Mandibular process of the mandibular arch posterior eye (G02b).	The mandibular process lays posterior to the eye for a long period. First, when a clear rostrad development of the mandibular process is recognizable, its position around the level of the posterior margin of the optic cup should be noted. For defining the level of the mandibular process in respect to the eye the ventral border of the telencephalic/diencephalic head region must be orientated horizontally.
P. Mandibular process (G02).	P3. Mandibular arch bud posterior lens (G02c).	The tip of the mandibular process is situated around the level of the posterior margins of scleral papillae, lens, iris and pupil.
P. Mandibular process (G02).	P4. Mandibular arch bud midline eye (G02d).	The tip of the mandibular process is located around the level of the optic (choroid) fissure, around the midline of the eye.
P. Mandibular process (G02).	P5. Mandibular arch bud anterior lens (G02e).	The tip of the mandibular process is located beyond the optic fissure and is situated around the level of the anterior borders of pupil, iris, lens and scleral papillae.
P. Mandibular process (G02).	P6. Mandibular arch bud anterior eye (G02f).	The tip of the mandibular process is situated at the level around the anterior border of the eye.
P. Mandibular process (G02).	P7. Mandibular arch bud level frontonasal groove (G02g).	The tip of the mandibular process is situated at the level of the frontonasal process.
P. Mandibular process (G02).	P8. Mandibular arch bud occlusion point = jaw closure (G02h).	The tip of the mandibular process comes in contact with the anterior part of the upper jaw.
Q. Postmandibular pharyngeal arches (G03).	Q1. Second arch bud (G03a).	The second pharyngeal arch, the hyoid arch occurs as a bud.
Q. Postmandibular pharyngeal arches (G03).	Q2. Hyoid flap (G03e).	The second (hyoid) arch develops an opercular flap that covers at least one pharyngeal slit.
Q. Postmandibular pharyngeal arches (G03).	Q3. Third arch bud (G03b).	The third pharyngeal arch occurs as a bud.		see Q1
Q. Postmandibular pharyngeal arches (G03).	Q4. Fourth arch bud (G03c).	The fourth pharyngeal arch occurs as a bud.		see Q1
Q. Postmandibular pharyngeal arches (G03).	Q5. Fifth arch bud (G03d).	The fifth pharyngeal arch occurs as a bud.		see Q1
R. Pharyngeal slits (G04).	R1. First pharyngeal slit (G04a).	The first pharyngeal slit, which is the lateral opening of the 1st pharyngeal pouch, occurs between the 1st and the 2nd pharyngeal arch.
R. Pharyngeal slits (G04).	R2. Second pharyngeal slit (G04b).	The second pharyngeal slit occurs between the 2nd and the 3rd pharyngeal arch.		see R1
R. Pharyngeal slits (G04).	R3. Third pharyngeal slit (G04c).	The third pharyngeal slit occurs between the 3rd and the 4th pharyngeal arch.		see R1
R. Pharyngeal slits (G04).	R4. Fourth pharyngeal slit (G04d).	The 4th pharyngeal slit occurs between the 4th and the 5th pharyngeal arch.		see R1
R. Pharyngeal slits (G04).	R5. Pharyngeal slits closed (G04e).	All pharyngeal slits are closed.
S. Urogenital papillae (G05).	S1. Urogenital papilla bud (G05a).	Between the hind limbs an urogenital papillae occurs as a distinct bud.
S. Urogenital papillae (G05).	S2. Urogenital papilla bud inconspicuous (G05b).	The urogenital organs are drawn into the body and the papillae becomes inconspicuous.
T. Neck (T01).	T1. Cervical flexure 90° (T01a).	During development of different species a 90° cervical flexure can occur and reverse several times. Only the first occurrence of a 90° cervical flexure is to be noted.
T. Neck (T01).	T2. Cervical flexure disappeared (T01b).	Only the final disappearance of the cervical flexure is to be noted.
T. Neck (T01).	T3. Wrinkles on neck (T01c).	The first occurrence of wrinkles on the dorsum of the neck.		see T2
T. Neck (T01).	T4. Nuchal fold (T01d).	A nuchal fold is forming in the rhombencephalic region.
U. Eyelids (A01).	U1. Lower lid appears (A01a).	First occurrence of the lower eyelid. This should not be confused with the remainder of the maxillary process that fused with the frontonasal process.
U. Eyelids (A01).	U2. Eyelid begun overgrow (A01b).	The lower eyelid has begun overgrowing the eye but does not reach the scleral papillae yet.
U. Eyelids (A01).	U3. Eyelid at level of scleral papillae (A01c).	The lower eyelid has grown dorsad up to the lower margin of the iris or is at least around the lower curvature of the scleral papillae.
U. Eyelids (A01).	U4. Eyelid ventral to lens (A01d).	The lower eyelid reaches the ventral margin of the lens.
U. Eyelids (A01).	U5. Eyelid covers eye (A01e).	The lower eyelid covers at least half of the eye/pupil. In reptiles the eye sometimes never closes completely.
U. Eyelids (A01).	U6. Membrana nictitans (A01f).	The membrana nictitans, the "third eyelid", occurs in the anterior angle of the eye.
U. Eyelids (A01).	U7. Eyelid open again (A01g).	The eyelids open after closure.
U. Eyelids (A01).	U8. Epitrichium covering eye (A01h).	In marsupials the eye is covered by the epitrichium.
V. Caruncle (A02).	V1. Caruncle appears (A02a).	The caruncle (egg tooth) is first visible as a medial calcification of the skin that covers the symphysis of the maxillaries and lies between the nasal openings. Later on it enlarges and fuses with the maxillaries to get a mechanical support for slashing the egg while hatching.
W. Ramphothecae (S01).	Ramphothecae appear (S01a).	Horny ramphothecae are sometimes described as "lips" or "labia" in literature and form distinct "beaks" in turtles or birds.
X. Carapace (S02a).	X1. Carapacial ridge (S02a).	On the lateral side of the trunk and between fore- and hind limb a distinct horizontal carapacial ridge occurs in turtle development.
X. Carapace (S02a).	X2. Longitudinal carapacial ridge (S02b).	The capacial ridge of turtles elongates beyond the roots of the limbs and forms a lateral longitudinal border of the already recognizable carapace.
X. Carapace (S02a).	X3. Carapace not reaching anterior (S02c).	The anterior border of the turtle carapace is not yet defined.
X. Carapace (S02a).	X4. Carapace clearly limited (S02d).	The turtle carapace is now clearly limited around its periphery.
X. Carapace (S02a).	X5. Carapace posterior projection (S02e).	The posterior carapacial edge of turtles projects beyond the root of the tail.
X. Carapace (S02a).	X6. Carapace irregular (S02f).	The turtle carapace becomes irregular around its periphery.
Y. Hair follicles (M04).	Y1. Back follicles (M04a).	Follicles occur on the back.
Y. Hair follicles (M04).	Y2. Belly follicles (M04b).	First follicles occur on the belly.		see Y1
Y. Hair follicles (M04).	Y3. Whole belly follicles (M04c).	Follicles occur on the whole belly.		see Y1
Y. Hair follicles (M04).	Y4. Forelimb follicles proximal (M04d).	Follicles occur on the proximal part of the forelimb.		see Y1
Y. Hair follicles (M04).	Y5. Forelimb follicles distal (M04e).	Follicles occur on the distal part of the forelimb.		see Y1
Y. Hair follicles (M04).	Y6. Hind limb follicles proximal (M04f).	Follicles occur on the proximal part of the hind limb.		see Y1
Y. Hair follicles (M04).	Y7. Hind limb follicles distal (M04g).	Follicles occur on the distal part of the hind limb.		see Y1
Y. Hair follicles (M04).	Y8. Tail follicles (M04h).	Follicles occur on the tail.		see Y1
Y. Hair follicles (M04).	Y9. Neck follicles (M04i).	Follicles occur on the neck (not throat).		see Y1
Y. Hair follicles (M04).	Y10. Head follicles (M04j).	Follicles occur on the head (except for snout region).		see Y1
Y. Hair follicles (M04).	Y11. Throat follicles (M04k).	Follicles occur on the throat.		see Y1
Y. Hair follicles (M04).	Y12. Snout follicles (M04l)	Vibrissal follicles on the snout occur.		see Y1
Z. Hairs (M01).	Z1. Back hairs (M01a).	Hairs occur on the back but not on the tail, the neck or the top of the head.
Z. Hairs (M01).	Z2. Belly hairs (M01b).	First hairs occur on the belly.		see Z1
Z. Hairs (M01).	Z3. Whole belly hairs (M01c).	The whole belly is covered by hairs.		see Z1
Z. Hairs (M01).	Z4. Forelimb hairs proximal (M01d).	The proximal part of the forelimb is covered by hairs.
Z. Hairs (M01).	Z5. Forelimb hairs distal (M01e).	The distal part of the forelimb is covered by hairs.		see Z4
Z. Hairs (M01).	Z6. Hind limb hairs proximal (M01g).	The proximal part of the hind limb is covered by hairs.
Z. Hairs (M01).	Z7. Hind limb hairs distal (M01h).	The distal part of the hind limb is covered by hairs.		see Z6
Z. Hairs (M01).	Z8. Tail hairs (M01f).	Hairs occur on the tail.		see Z6
Z. Hairs (M01).	Z9. Neck hairs (M01i).	Hairs occur on the neck (not throat).
Z. Hairs (M01).	Z10. Head hairs (M01j).	Hairs occur on the head (except for snout region).		see Z6
Z. Hairs (M01).	Z11. Throat hairs (M01k).	Hairs occur on the throat.		see Z6
Z. Hairs (M01).	Z12. Snout hairs (M01k).	Vibrissal hairs occur on the snout.		see Z6
AA. Trunk coiling (M02).	AA1. Trunk coiling (M02a).	The trunk is coiled by about 180°.
AA. Trunk coiling (M02).	AA2. Trunk coiling disappeared (M02b).	The 180° coiling of the trunk finally disappears.		see AA1
AB. Marsupium (M03).	AB1. Marsupium anlage (M03a).	The pouch (marsupium) develops as a distinct field on the belly.
AB. Marsupium (M03).	AB2. Marsupium dent (M03b).	The marsupial anlage, previously visible as a distinct field, now dents to a concave buckling into the belly.
AC. Monotreme beak (MO01).	AC1. Monotreme beak (MO01a).	The jaws narrow to a distinct beaklike snout and are clearly separated from the head, i.e. by a skin fold.
AD. Hemipenes (SQ01).	AD1. Hemipenes appears (SQ01a).	The everted hemipenes are seen as paired structures sticking out on each side of the cloaca. Depending on the taxa, different forms are possible. The hemipenes are only found in males.
AD. Hemipenes (SQ01).	AD2. Hemipenes disappeared (SQ01b).	The hemipenes are inverted and thus no longer visible from the outside.
AE. Mammary anlage (M04).	AE1. Mammary anlage (M04a).	The mammary organ is visible externally.
AF. Tongue protruding (T02).	AF1. Tongue protruding (T02a).	The tongue protrudes over the borders of the snout.
AG. Patagium (flying membrane) (M05).	AG1. Plagiopatagium (M05a).	At the flank of chiropterans, the anlage of a plagiopatagium is present between forelimb and hind limb.
AG. Patagium (flying membrane) (M05).	AG2. Uropatagium (M05b).	Between the hind limbs of chiropterans, the anlage of an uropatagium is present.		see AG1
AG. Patagium (flying membrane) (M05).	AG3. Propatagium (M05c).	N.N.		see AG1
AG. Patagium (flying membrane) (M05).	AG4. Chiropatagium (M05d).	N.N.		N.N.
AG. Patagium (flying membrane) (M05).	AG5. Free thumb (M05e).	N.N.		N.N.
AG. Patagium (flying membrane) (M05).	AG6. Calcar (M05f).	N.N.		N.N.
AH. Pigmentation.	AH1. Claw pigmentation.	N.N.		N.N.
AH. Pigmentation.	AH2. Body pigmentation.	N.N.		N.N.

References

1. ^ Werneburg (2009). A Standard System to Study Vertebrate Embryos. PLoS ONE 4(6): e5887. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0005887
2. Werneburg and Spiekman (2016). Mammalian embryology and organogenesis. From gametes to weaning. In: Zachos F., Asher R. (eds.). Mammalia. Series: The Handbook of Zoology / Handbuch der Zoologie. De Gruyter, Berlin
3. ^ Werneburg I and Sánchez-Villagra MS (2009). Timing of organogenesis support basal position of turtles in the amniote tree of life. BMC Evolutionary Biology, 9:82 http://www.biomedcentral.com/1471-2148/9/82
4. Werneburg I, Hugi J, Müller J, Sánchez-Villagra MS (2009). Embryogenesis and ossification of Emydura subglobosa (Testudines, Pleurodira, Chelidae) and patterns of turtle development. Developmental Dynamics, Volume 238, Issue 11 Pages 2770-2786 http://onlinelibrary.wiley.com/doi/10.1002/dvdy.22104/abstract
5. Nunes Silva R and Sobral Sampaio F (2014). Immunoreactivity of Mel1a-like melatonin receptor and NRH: Quinone reductase enzyme (QR2) in testudine whole embryo and in developing whole retinas Trends in Developmental Biology 8:39-46.
6. ^ Polachowski KM and Werneburg I (2013). Late embryos and bony skull development in Bothropoides jararaca (Serpentes, Viperidae). Zoology
7. Roscito and Rodriges (2012). Embryonic development of the fossorial gymnophthalmid lizards Nothobachia ablephara and Calyptommatus sinebranchiatus. Zoology 115:302-318
8. Werneburg et al. (2015). Bony skull development in the Argus monitor (Squamata, Varanidae, Varanus panoptes) with comments on developmental timing and adult anatomy. Zoology 118(4):255-280
9. Ollonen, J., Da Silva, F.O., Mahlow, K. and Di-Poï, N., 2018. Skull development, ossification pattern, and adult shape in the emerging lizard model organism Pogona vitticeps: a comparative analysis with other squamates. Frontiers in physiology, 9, p.278.
10. ^ Werneburg I and Sánchez-Villagra MR (2011). The early development of the echidna, Tachyglossus aculeatus (Mammalia: Monotremata) and patterns of mammalian development. Acta Zoologica. 82(1) http://onlinelibrary.wiley.com/doi/10.1111/j.1463-6395.2009.00447.x/abstract
11. ^ Werneburg I, Laurin M, Koyabu D, Sánchez-Villagra (2016). Evolution of organogenesis and the origin of altriciality in mammals. Evolution and Development
12. González B, Soria-Escobar AM, Rojas-Díaz V, Pustovrh MC, Monsalve LS, Rougier GW (2020). The embryo of the silky shrew opossum, Caenolestes fuliginosus (Tomes, 1863): First description of the embryo of Paucituberculata. Journal of Morphology 2020: 1-12
13. ^ Werneburg I, Tzika AC, Hautier L, Asher RJ, Milinkovitch MC, Sánchez-Villagra MR (2013). Development and embryonic staging in non-model organisms: the case of an afrotherian mammal. The Journal of Anatomy 222:2-18
14. ^ Taro Nojiri, Dai Fukui, Ingmar Werneburg, Takashi Saitoh, Hideki Endo, Daisuke Koyabu (2021). Embryonic staging of bats with special reference to Vespertilio sinensis and its cochlear development. Development Dynamics, DOI10.1002/dvdy.325

• Embryology