Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae), Chondrocranium at the Early Stages of Embryogenesis
The initial developmental stages of grass snake’s, Natrix natrix Linnaeus, 1758 chondrocranium are described. Three paired structures form the floor of N. natrix’s neurocranium: cranial trabeculae, polar cartilages, and parachordals. The primordiums of polar cartilages and their independent centers...
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Інститут зоології ім. І.І. Шмальгаузена НАН України
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irk-123456789-1094682016-11-30T03:02:25Z Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae), Chondrocranium at the Early Stages of Embryogenesis Sheverdyukova, H.V. Морфология The initial developmental stages of grass snake’s, Natrix natrix Linnaeus, 1758 chondrocranium are described. Three paired structures form the floor of N. natrix’s neurocranium: cranial trabeculae, polar cartilages, and parachordals. The primordiums of polar cartilages and their independent centers of chondrification are identified at the stage 26 of development for the first time for N. natrix and snakes in general. The participation of these structures in the formation of crista sellaris and carotid foramina is proved. Описаны начальные этапы развития хрящевого черепа Natrix natrix Linnaeus, 1758. Три парные структуры: черепные трабекулы, полярные хрящи и парахордалии образуют дно нейрокраниума N. natrix. Зачатки полярных хрящей и их независимые центры охрящевения определены на 26-й стадии развития впервые для N. natrix и змей в целом. Доказано участие этих структур в формировании crista sellaris и сонных отверстий. 2012 Article Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae), Chondrocranium at the Early Stages of Embryogenesis / H.V. Sheverdyukova // Вестник зоологии. — 2012. — Т. 46, № 5. — С. 461–468. — Бібліогр.: 16 назв. — англ. 0084-5604 DOI 10.2478/v10058-012-0038-5 http://dspace.nbuv.gov.ua/handle/123456789/109468 591.471.4:598.115.31 en Вестник зоологии Інститут зоології ім. І.І. Шмальгаузена НАН України |
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Морфология Морфология |
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Морфология Морфология Sheverdyukova, H.V. Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae), Chondrocranium at the Early Stages of Embryogenesis Вестник зоологии |
| description |
The initial developmental stages of grass snake’s, Natrix natrix Linnaeus, 1758 chondrocranium are described. Three paired structures form the floor of N. natrix’s neurocranium: cranial trabeculae, polar cartilages, and parachordals. The primordiums of polar cartilages and their independent centers of chondrification are identified at the stage 26 of development for the first time for N. natrix and snakes in general. The participation of these structures in the formation of crista sellaris and carotid foramina is proved. |
| format |
Article |
| author |
Sheverdyukova, H.V. |
| author_facet |
Sheverdyukova, H.V. |
| author_sort |
Sheverdyukova, H.V. |
| title |
Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae), Chondrocranium at the Early Stages of Embryogenesis |
| title_short |
Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae), Chondrocranium at the Early Stages of Embryogenesis |
| title_full |
Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae), Chondrocranium at the Early Stages of Embryogenesis |
| title_fullStr |
Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae), Chondrocranium at the Early Stages of Embryogenesis |
| title_full_unstemmed |
Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae), Chondrocranium at the Early Stages of Embryogenesis |
| title_sort |
polar cartilages and formation of crista sellaris in grass snake, natrix natrix (ophidia, colubridae), chondrocranium at the early stages of embryogenesis |
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Інститут зоології ім. І.І. Шмальгаузена НАН України |
| publishDate |
2012 |
| topic_facet |
Морфология |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/109468 |
| citation_txt |
Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae), Chondrocranium at the Early Stages of Embryogenesis / H.V. Sheverdyukova // Вестник зоологии. — 2012. — Т. 46, № 5. — С. 461–468. — Бібліогр.: 16 назв. — англ. |
| series |
Вестник зоологии |
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AT sheverdyukovahv polarcartilagesandformationofcristasellarisingrasssnakenatrixnatrixophidiacolubridaechondrocraniumattheearlystagesofembryogenesis |
| first_indexed |
2025-07-07T23:09:02Z |
| last_indexed |
2025-07-07T23:09:02Z |
| _version_ |
1837031483716403200 |
| fulltext |
UDC 591.471.4:598.115.31
POLAR CARTILAGES AND FORMATION
OF CRISTA SELLARIS IN GRASS SNAKE, NATRIX NATRIX
(OPHIDIA, COLUBRIDAE), CHONDROCRANIUM AT THE
EARLY STAGES OF EMBRYOGENESIS
H. V. Sheverdyukova
Schmalhausen Institute of Zoology, NAS of Ukraine,
B. Chmielnitsky str., 15, Kyiv, 01601 Ukraine
E-mail: hstramontana@gmail.com
Received 6 March 2012
Accepted 14 September 2012
Polar Cartilages and Formation of Crista Sellaris in Grass Snake, Natrix natrix (Ophidia, Colubridae),
Chondrocranium at the Early Stages of Embryogenesis. Sheverdyukova H. V. – The initial developmen-
tal stages of grass snake’s, Natrix natrix Linnaeus, 1758 chondrocranium are described. Three paired struc-
tures form the floor of N. natrix’s neurocranium: cranial trabeculae, polar cartilages, and parachordals.
The primordiums of polar cartilages and their independent centers of chondrification are identified at the
stage 26 of development for the first time for N. natrix and snakes in general. The participation of these
structures in the formation of crista sellaris and carotid foramina is proved.
Ke y wo r d s: polar cartilage, crista sellaris, chondrocranium, embryonic development, carotid forami-
na, Natrix natrix.
Ïîëÿðíûå õðÿùè è ôîðìèðîâàíèå crista sellaris â õðÿùåâîì ÷åðåïå óæà îáûêíîâåííîãî, Natrix natrix
(Ophidia, Colubridae), íà ðàííèõ ñòàäèÿõ ýìáðèîãåíåçà. Øåâåðäþêîâà À. Â. – Îïèñàíû íà÷àëüíûå
ýòàïû ðàçâèòèÿ õðÿùåâîãî ÷åðåïà Natrix natrix Linnaeus, 1758. Òðè ïàðíûå ñòðóêòóðû: ÷åðåïíûå
òðàáåêóëû, ïîëÿðíûå õðÿùè è ïàðàõîðäàëèè îáðàçóþò äíî íåéðîêðàíèóìà N. natrix. Çà÷àòêè ïîëÿð-
íûõ õðÿùåé è èõ íåçàâèñèìûå öåíòðû îõðÿùåâåíèÿ îïðåäåëåíû íà 26-é ñòàäèè ðàçâèòèÿ âïåð-
âûå äëÿ N. natrix è çìåé â öåëîì. Äîêàçàíî ó÷àñòèå ýòèõ ñòðóêòóð â ôîðìèðîâàíèè crista sellaris
è ñîííûõ îòâåðñòèé.
Êëþ÷åâûå ñëîâà: ïîëÿðíûå õðÿùè, crista sellaris, ñîííûå îòâåðñòèÿ, ýìáðèîíàëüíîå ðàçâèòèå,
õðÿùåâîé ÷åðåï, Natrix natrix.
Introduction
The trabeculae cranii and basal plate (planum basale) are the first elements of the chondrocranium, which
appear in the cranial embryogenesis of vertebrates, particularly snakes, and form the floor of neurocranium (Parker,
1878; Bäckström, 1931; Chekanovskaya, 1936; Bellairs, Kamal, 1981). Trabeculae appear in the form of sep-
arate paired bars. Later, their oral ends merge into the trabecula communis, and the aboral ones extend and
merge with the oral-lateral edges of the basal plate. Triangular fenestra, limited laterally by the trabeculae, oral-
ly by the trabecula communis, and aborally by the oral edge of the basal plate, is called the pituitary fenestra
(fenestra hypophysale).
The way of basal plate’s primordium appearance in snake embryogenesis remains controversial. A com-
mon statement about initially independent primordiums of parachordals merging later into a basal plate in all
vertebrates (de Beer, 1937), is questioned by researchers who studied the embryogenesis of a snake skull
(Bäckström, 1931; Chekanovskaya, 1936; Bellairs, Kamal, 1981; Haluska, Alberch, 1983). The authors
described a single primordium of the basal plate in the form of mesenchymal tissue, enveloping the notochord
at the early stages of development.
According to most researchers, the fenestra basicranialis in snakes’ chondrocranium is formed at the later
stages of embryogenesis as a result of the cartilage resorption in the basal plate; it is limited by the oral por-
tion of the basal plate, which is called crista sellaris. Thus, crista sellaris is a transverse bar that separates the
Vestnik zoologii, 46(5): e-32—e-39, 2012
DOI 10.2478/v10058-012-0038-5
Unauthenticated
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pituitary and the basicranial fenestrae. N. natrix and Lamprophis inornatus (Colubridae) have foramina in the
lateral parts of the crista sellaris for the internal carotid arteries – carotid foramina – to pass (Parker, 1878;
Bäckström, 1931; Chekanovskaya, 1936; Pringle, 1954).
In most existing works structures of chondrocranium are described at fairly late stages, when the floor
of neurocranium is already fully developed. There are only a few studies in which primordiums and formation
of the basic structures of the snakes’ chondrocranium (trabeculae and basal plate) at the initial stage of its devel-
opment are described (Parker, 1878; Bäckström, 1931; Chekanovskaya, 1936).
The aim of our study was to determine the timing and the way of appearance and development of the
chondrocranium basic elements at the early stages of normal N. natrix embryogenesis.
Material and methods
The material was collected in June—July 2010—2011. Pregnant N. natrix females were caught in their habi-
tat and placed in terrariums, where they oviposited. The eggs were incubated in wetted vermiculite at a tem-
perature of 27—30° C. Two eggs of each clutch were taken every day. At a point of egg laying N. natrix embryos
are at stages 27+ of normal development. The stages of embryo development were determined by a table of
normal development, worked out for Thamnophis sirtalis (Colubridae) (Zehr, 1962).
Embryos at 25—27th stages of development were extracted during caesarean section. The operative tech-
nique follows H. Clark (1937). Immobilization and anesthesia were performed according to the method intro-
duced by D. B. Vasiliev, A. M. Timerina (2000) especially for reptiles. Medetomidin 50—70 mg/kg was used
as an anesthetic. After oviposition, performed operations and rehabilitation all the females were released in their
place of their capture.
The embryonic material was fixed in 4 % formalin solution. The dry-out material was placed into paraf-
fin, thereafter serial histological 5—7 microns thick sections were prepared. The sections were stained with alcian-
blue – hematoxylin – eosin.
Twenty-five embryos at stages 25—32 of normal development were examined. A few embryos were inves-
tigated at one stage. The stage on which the embryo development has substantially advanced, but not yet reached
the next one is marked with the sign “+”.
Photographs of microsections were made with a microscope Zeiss Axio Imager M1 and software Zeiss
Axio Vision v. 4.63 in the Centre of Common Access to Equipment at the I. I. Schmalhausen Institute of Zoology,
NAS of Ukraine.
Cleared and stained embryos were prepared as described by E. Simons, D. R. Van Horn (1971). The
whole mounts do not show structures, where the processes of chondrification is uncompleted, so some stages
of development are illustrated by cleared and stained embryos at later stages.
Result
S t a g e 2 5
The first primordiums of the future elements of Natrix natrix chondrocranium
appear at the stage 25 of normal development: ventral to the forebrain and ventro-medi-
al to eyes, in the general mass of friable mesenchyme, paired clusters of mesenchymal
cells in the form of parallel beams are seen (fig. 1, a). These are the mesenchymal pri-
mordiums of cranial trabeculae. Their aboral ends lay in front of carotid arteries.
At this stage, at the sides of the notochord head end the primordiums of parachordals
appear in the form of elongated mesenchymal plates (fig. 1, b). As a result of the well-
marked at this stage cervical flexure, the primordiums of trabeculae and parachordals are
almost orthogonally situated.
S t a g e 2 6
The mesenchymal primordiums of another structures are visible dorsal to the para-
chordal primordiums, which tend to merge with oral ends of the latter. They are situat-
ed in one plane with the primordiums of parachordals. At the pituitary level their oral ends
are bent towards each other; they are medial to the carotid arteries (fig. 2). Based on the
topography of these mesenchymal structures, we believe them to be the primordiums of
the polar cartilages (cartilago polaris), described in some cartilaginous fishes, bony fish-
es and birds (de Beer, 1937; Kovtun et al., 2008). The head end of the notochord, the
primordiums of polar cartilages and of prachordals are situated ventrally to the hindbrain.
At this stage the cervical flexure remains, so the primordiums of polar cartilages and
of parachordals are orthogonal to the primordiums of the cranial trabeculae.
e-33 H. V. Sheverdyukova
Unauthenticated
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e-34Polar Cartilages and Formation of Crista Sellaris in Grass Snake’s, Natrix natrix...
Fig. 1. Cross-section of N. natrix embryo’s head at the stage 25 of the development: 1 – forebrain; 2 – eye;
3 – mesenchymal primordiums of the trabeculae cranii; 4 – notochord; 5 – mesenchymal primordiums of
the parañhordals; 6 – spinal cord; 7 – hindbrain.
Ðèñ. 1. Ïîïåðå÷íûå ñðåçû ãîëîâû ýìáðèîíà N. natrix 25-é ñòàäèè ðàçâèòèÿ: 1 – ïåðåäíèé ìîçã; 2 –
ãëàç; 3 – ìåçåíõèìíûå çàêëàäêè ÷åðåïíûõ òðàáåêóë; 4 – õîðäà; 5 – ìåçåíõèìíûå çàêëàäêè ïàðàõîð-
äàëèé; 6 – ñïèííîé ìîçã; 7 – çàäíèé ìîçã.
Fig. 2. Cross-section of N. natrix embryo`s head at the stage 26 of the development: 1 – midbrain; 2 – eye;
3 – carotid arteries; 4 – mesenchymal primordiums of the polar cartilages; 5 – mesenchymal primordiums
of the parahordals.
Ðèñ. 2. Ïîïåðå÷íûå ñðåçû ãîëîâû ýìáðèîíà N. natrix 26-é ñòàäèè ðàçâèòèÿ: 1 – ñðåäíèé ìîçã; 2 – ãëàç;
3 – ñîííûå àðòåðèè; 4 – ìåçåíõèìíûå çàêëàäêè ïîëÿðíûõ õðÿùåé; 5 – ìåçåíõèìíûå çàêëàäêè ïàðà-
õîðäàëèé.
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S t a g e 2 7
The cervical flexure is less marked. As a result, the structures of the future chon-
drocranium, described above, changed their relative positions: the angle between the pri-
mordiums of the trabeculae and the polar cartilages becomes blunt; the primordiums of
the polar cartilages surround the internal carotid artery from the aboral side. The medi-
al parts of the polar cartilages’ primordiums are merged with the oral ends of the para-
chordals’ primordiums at right angles in the transverse plane. At this stage the process-
es of chondrification are visible in all three structures: trabeculae, polar cartilages and para-
chodrals (fig. 3). Each of them has an independent center of chondrification. The
processes of chondrification are seen in the medial zone of the trabeculae’ and polar car-
tilages’ primordiums. The aboral halves of the parachordals are merged, forming a basal
plate (fig. 3, c). The processes of chondrification are more marked in the basal plate, that
is chondrification of this structure proceeds in the aboral-oral direction.
S t a g e 2 7+
The aboral mesenchymal ends of the trabeculae merge with the lateral mesenchy-
mal edges of the polar cartilages approximately orthogonally (in the transverse plane)
e-35 H. V. Sheverdyukova
Fig. 3. Cross-section of N. natrix embryo`s head at the stage 27 of the development at different levels: 1 –
forebrain; 2 – eye; 3 – primordiums of the trabeculae cranii; 4 – rostrum; 5 – Meckel’s cartilage; 6 – pri-
mordiums of the polar cartilages; 7 – quadrate cartilage; 8 – ganglion of the trigeminal nerve; 9 – carotid
arteries; 10 – semicircular canals; 11 – basal plate; 12 – hindbrain; 13 – oculomotor nerve.
Ðèñ. 3. Ïîïåðå÷íûå ñðåçû ãîëîâû ýìáðèîíà N. natrix 27-é ñòàäèè ðàçâèòèÿ: 1 – ïåðåäíèé ìîçã; 2 –
ãëàç; 3 – çàêëàäêè òðàáåêóë; 4 – ðîñòðóì; 5 – Ìåêêåëåâ õðÿù; 6 – çàêëàäêè ïîëÿðíûõ õðÿùåé; 7 –
êâàäðàòíûé õðÿù; 8 – ãàíãëèé òðîéíè÷íîãî íåðâà; 9 – ñîííûå àðòåðèè; 10 – ïîëóêðóæíûå êàíàëû;
11 – áàçàëüíàÿ ïëàñòèíêà; 12 – çàäíèé ìîçã; 13 – ãëàçîäâèãàòåëüíûé íåðâ.
Unauthenticated
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(fig. 4, a). The medial edges of the polar cartilages are merged with the oral ends of para-
chordals also orthogonally so that the boundary between them is impossible to distinguish.
S t a g e 2 8
All the basic structures, described above are completely cartilaginous. The aboral ends
of the trabeculae are merged with the lateral edges of the polar cartilages. The places of
their junction are located lateral to the internal carotid arteries: at the point where the
latter bend dorsally, going to the brain. At this stage the chondrifying processes from the
medial edges of the polar cartilages depart in the oral direction. They surround the inter-
nal carotid arteries from the medial side, forming the so-called carotid incisures (fig. 4, b).
S t a g e 2 9
The processes of the polar cartilages are merged with the medial surface of the tra-
beculae, closing the passages for the internal carotid arteries and forming the carotid foram-
ina (fig. 4, c). Transverse bar, the so called crista sellaris, connects the medial borders
of the fully formed carotid foramina. The initial stage of its chondrification is marked.
The crista sellaris divides the space between the beams formed by the trabecula, the polar
cartilage and the parahordalia’s oral end on each side into two fenestrae: pituitary and
basicranial fenestrae. Thus, the pituitary fenestra is triangular in shape: it is limited by
e-36Polar Cartilages and Formation of Crista Sellaris in Grass Snake’s, Natrix natrix...
Fig. 4. Cleared and stained cranium of N. natrix embryo in dorsal view: a – stage of development 29; b –
30+; c – 31; d – 32; e – the future crista sellaris region in chondrocranium of embryo Amia calva (graphic
reconstruction after Pehrson, 1922): 1 – trabecula; 2 – polar cartilage; 3 – pituitary fenestra; 4 – carotid
incisure; 5 – carotid foramen; 6 – crista sellaris; 7 – basicranial fenestra; 8 – basal plate; 9 – notochord.
Ðèñ. 4. Òîòàëüíûé ïðåïàðàò ÷åðåïà ýìáðèîíà N. natrix âèä ñâåðõó: a – 29; b – 30+; c – 31; d – 32-ÿ
ñòàäèÿ ðàçâèòèÿ; e – îáëàñòü ôîðìèðîâàíèÿ áóäóùåé crista sellaris â õðÿùåâîì ÷åðåïå ýìáðèîíà Amia
calva (ãðàôè÷åñêàÿ ðåêîíñòðóêöèÿ èç Ïåðñîí, 1922): 1 – òðàáåêóëà; 2 – ïîëÿðíûé õðÿù; 3 – ãèïî-
ôèçàðíîå îêíî; 4 – ñîííàÿ âûðåçêà; 5 – ñîííîå îòâåðñòèå; 6 – crista sellaris; 7 – áàçèêðàíèàëüíîå
îêíî; 8 – áàçàëüíàÿ ïëàñòèíêà; 9 – õîðäà.
Unauthenticated
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the trabecula communis orally, by the trabeculae laterally and by the crista sellaris abo-
rally. The basicranial fenestra is limited by the crista sellaris orally, by the free ends of
the parachordals laterally and by the basal plate aborally (fig. 4, d).
At the later stages of development, the above mentioned structures grow and the pro-
cesses of chondrification become more intensive in them. No other visible changes were
observed. The chondrification of the crista sellaris ends at the stage of development 32.
Discussion
As already mentioned, there is no consensus on the way the basal plate’s primordi-
ums emerge in snakes’ chondrocranium. We observed a pair of the parachordal primordi-
ums at Stage 25, which confirms the E. Gaupp’s (1906) and G. R. de Beer’s (1937) find-
ings. We were unable to determine whether each parachordal has an independent cen-
ter of chondrification or the chondrification occurs after the mesenchymal aboral ends
of parachordal primordiums merge, i.e., already in the basal plate. We believe that the
parachordals merge at the mesenchymal stage around the head notochord, forming the
so-called “enveloping mass” of mesenchymal cells, which most researchers described as
an unpaired primordium of snakes’ basal plate (Bäckström, 1931; Chekanovskaya, 1936;
Bellairs, Kamal, 1981). According to our findings, the basal plate is formed by the con-
fluence of the aboral ends of parachordals; the oral ends remain free, and the space between
them is a future basicranial fenestra. As opposed to the authors mentioned above, we
observed no resorption of the basal plate cartilage.
The polar cartilages we discovered in N. natrix between trabeculae and parahordals
are described for the first time in snakes. The only mention of these structures in the snakes’
chondrocranium is found in H. R. Srinivasahar (1955), who studied the embryo of Vipera
russelii at one late stage, when the chondrocranium is fully formed. The author took note
of the characteristic flexure of the aboral parts of trabeculae and its localization (lateral
to the internal carotid arteries). At that stage it was impossible to determine, whether these
zones were formed by separate cartilages, located between the trabeculae and parahordals,
or they are actually trabeculae. However, the author suggested that if these zones appear
as separate cartilages, they should be considered as polar cartilages, described in fishes
and birds. We managed to find independent primordiums of these structures in N. natrix
that confirms H. R. Srinivasahar’s assumption. For the first time we discovered the pri-
mordiums of polar cartilages at the stage 26 of development, when they begin to merge
with the oral ends of the parahordals’ primordiums; an independent center of chondri-
fication in these structures is observed at the stage 27 of development. Thus, the floor of
the cartilage neurocranium of N. natrix is formed by three paired structures – trabecu-
lae, polar cartilages and parahordals – probably homologous to those found in fishes and
birds. The same as in fishes and birds, the polar cartilages in N. natrix merge with tra-
beculae lateral to the internal carotid arteries. In fish embryos at the late stages of devel-
opment, the individual polar cartilage could not be discerned, but the researchers
(Pehrson, 1922; de Beer, 1937) assumed that the place of their junction with the trabec-
ulae is located lateral to the carotid arteries. In N. natrix at stage 27+ the polar carti-
lages do not look as separate structures, too.
The process of crista sellaris’ formation in snakes was described in the works of those
authors who managed to study the early stages of embryo development (Parker, 1878;
Bäckström, 1931). The authors, who claimed that basicranial fenestra is the result of car-
tilage’s resorption of the basal plate identified the oral margin of the latter as crista sell-
aris (Backstrom, 1931; Bellairs, Kamal, 1981). W. K. Parker (1878) described another way
of crista sellaris formation. He believed that the transverse bar, called by him “post-pitu-
itary bridge”, is formed by two processes that deviate medially toward each other from the
aboral ends of the trabeculae (in the place of their junction with parachordals) and merge.
e-37 H. V. Sheverdyukova
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We found that the crista sellaris in N. natrix is formed by the polar cartilages and
their processes, not the processes of the trabeculae, as W. K. Parker considered.
The transverse bar, dividing the pituitary and basicranial fenestrae is described in all
vertebrates as crista sellaris in amphibians and snakes, acrochordal cartilage in lizards,
turtles and birds, dorsum sellae in mammals (de Beer, 1937). Different names of this struc-
ture in fishes are mentioned: crista sellaris (Squalus acanthias), postpituitary commissure
(Scyllium canicula), dorsum sellae (Torpedo ocellata), acrochordal cartilage (Callorhynchus
antarcticus), prootic bridge (Lepisosterus osseus, Salmo fario, Exocoetus, Anguilla vulgaris,
Gadus merlangus, Neoceratodus). The different ways of its formation are described in the
fishes: in Lepisosterus osseus and Salmo fario prootic bridge appears as an independent
structure. In Acipenser ruthenus as in amphibians the crista sellaris is formed by the con-
fluence of the oral parahordals’ ends (de Beer, 1937).
The process of crista sellaris formation we observed in N. natrix is similar with those
of some cartilaginous fishes whose polar cartilages’ processus merged with each other form-
ing transverse bar (de Beer, 1937).
There is no consensus on how the snakes’ carotid foramina are formed. E. Gaupp
(1906) believed that the carotid foramina in snakes are formed mainly by trabeculae and
lateral processes, departing from their bottom and connecting to the basal plate. According
to him, trabeculae surround carotid arteries medially. K. Beckstrom (1931) believed that
the trabeculae surround carotid arteries laterally and the latter are medially surrounded
by medial trabeculae’ processes.
Among Ophidia the carotid foramina are described only in two species of the fam-
ily Colubridae: Natrix natrix (Parker, 1879; Bäckström, 1931; Chekanovskaya, 1936) and
Lamprophis inornatus (Pringle, 1954). J. A. Pringle believed its presence is a primitive fea-
ture of ñolubrid snakes, inherited from ancestral forms. It should be noted that in most
other studied snakes the orally open notches, not enclosed foramina, are described for
the passage of the internal carotid arteries. These notches are located at the oral edge of
the crista sellaris, where they merge with trabeculae (Pringle, 1954; Srinivasachar, 1955;
Bellairs, Kamal, 1981; Haluska, Alberch, 1983). We observed such carotid notches in N.
natrix at the stage 28 of development. According to our findings, they are formed by the
polar cartilages and by their processes. The similar carotid notches are described in Amia
calva (Pehrson, 1922) (fig. 4, e).
In the literature, different ways of the crista sellaris’ (acrochordal cartilage’s) for-
mation are described in lizards. G. R. de Beer (de Beer, 1937) discovered processes, going
medially from the junction of trabeculae and parachordals, which merge with each
other. Such a way of the crista sellaris’ formation is similar to the way of formation of
the «post-pituitary bridge», described by W. K. Parker (1878) in N. natrix. According to
some authors (Bellairs, Kamal, 1981), crista sellaris is the oral edge of the basal plate:
crista sellaris is recognized as a separate structure only after the formation of the basi-
cranial fenestra as a result of cartilage resorption. According to A. N. Yarygin (2009),
the acrochordal cartilage in Lacerta agilis appears originally as an independent structure.
The similar way of the acrochordal cartilage’s formation is described in turtles (de Beer,
1937; Bellairs, Kamal, 1981) and birds (de Beer, 1937; Kovtun et. al., 2008).
Some authors (Rieppel, Zaher, 2001) have called crista sellaris of snakes “the acro-
chordal cartilage”, although, as follows from our investigation, the way of its formation
differs from that of turtles, lizards and birds. According to our findings, an “ancient” way
of the crista sellaris’ formation in N. natrix is preserved. It should be homologous to that
described in some fishes, so we consider crista sellaris to be an analogous structure to the
acrochordal cartilage of turtles, lizards and birds.
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e-38Polar Cartilages and Formation of Crista Sellaris in Grass Snake’s, Natrix natrix...
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Bellairs A. d’A. Kamal A. M. The chondrocranium and the development of the skull in recent reptiles // Biology
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e-39 H. V. Sheverdyukova
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