Paper - A Human Embryo of Thirteen Somites
Embryology - 19 Apr 2024    Expand to Translate
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Wallin IE. A Human Embryo of Thirteen Somites Am J Anat. 1913;
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A Human Embryo Of Thirteen Somites
Ivan E. Wallin
Anatomical Laboratory, Bellevue Hospital Medical College, New York City
Seven Figures
The embryo which forms the basis of this work was given to
me by Dr. Rudolph Boencke in the spring of 1911. It has been
placed in the collection of the Department of Anatomy at the
University and Bellevue Hospital Medical College and is called
embryo no. 4.
The embryo was aborted two weeks after the last menstrual
period. There was no record of coitus. After fixation and with
the amnion intact the embryo measured 2.3 mm. in length. It
was cut into transverse sections 5 u in thickness, and stained
with iron‘ haematoxylin. The embryo yielded 287 sections.
Wax plate reconstructions were made of the complete embryo,
the heart, the foregut, also of the caudal part of the medullary
tube with the hind-gut and the belly stalk vessels. A graphic
reconstruction was made representing the embryo cut in the
mid-sagittal plane. All the reconstructions were made at a magnification of 200.
The embryo appears to be normal in every respect and the
following points of structure have been determined.
External Form
In its general configuration this embryo is very similar to Pfannenstiel III described by Low (’08). The body has a regu- lar dorso-ventral curve and has a slight twist so that the head is situated to the right of the mid-sagittal plane. The yolk sac communicates with the primitive gut by means of an extensive yolk stalk. The latter has its greatest diameter in the cephalo~caudal direction and its lateral width is greatest at the cephalic end. Caudal and to the right of the yolk stalk the belly stalk leaves the embryo passing ventrally and curving to the right and caudad. Lateral to the yolk stalk the embryonic coelom has an extensive communication with the extra-embryonic coelom.
The heart produces a prominent bulging of the right side of
the body immediately caudad to the head. The most prominent
part of the bulging marks the flexure in the heart tube between
the bulbus cordis and the ventricle. ‘The neck flexure has not
advanced to any prominent degree. There are two prominences
on the dorsal surface of the head region, one at the cephalic end
of the mid—brain and the other at the cephalic end of the hindbrain. Caudally the body curves gradually in a ventral direction. There is no distinct caudal flexure.
The medullary tube is open to the exterior at both ends. The
cephalic neuropore exhibits an unusual appearance for an embryo
of this age. It is Very wide and gives a great breadth to the
head when viewed from the ventral aspect. The lateral lips of
this neuropore curve dorsally and form the ventral boundary of
a deep groove which is directed cephalo—caudally. The caudal
end of this groove runs into the stomodeum. This part of the
nervous system which represents the forebrain has not kept
apace with the development of the remainder of the tube. It
apparently is a persistence of the condition which is present in
an earlier stage of development. Eternod’s (’95) embryo of
eight somit'es and the embryo of seven somites described by
Dandy (’10) exhibit cephalic neuropores which appear to be in
about the_same stage of development.
There are no indications of otic invaginations. Two pairs of entodermal pouches are in contact with the ectoderm. The points of contact are indicated on the surface by shallow depressions. In figure 2 their positions have been indicated on the surface by broken lines.
The amnion lies close on to the body of the embryo. The
head fold crosses the ventral aspect of the heart at about its
middle. The lateral folds follow the lateral lips of the coelom.
The tail fold is situated on the dorsal aspect of the belly stalk.
Nervous System
The nervous system has not proceeded Very far in its differentiation. The brain flexures do not agree With the His models of this stage, but correspond more to the older embryos described by Thompson (’07) and van den Broek (’11). The most distal portion representing the forebrain is still open and is bent almost at right angles to the mid-brain. The long axis of the forebrain lies in a cephalo-caudal plane and almost parallel with the long axis of the hind-brain. The most cephalic point of the nervous system_ is thus represented by the junction of the forebrain and the mid-brain. Near the caudal extremity of the forebrain there is a thickening together with an evagination of the brain ectoderm. This evagination is almost in contact with the ectoderm of the stomodeum and undoubtedly represents the infundibulum. Cephalad to the infundibulum and about in the middle of the lateral expansions of the cephalic neuropore there is a slight depression of the ectoderm on each side which represents the beginning of the optic Vesicles.
The mid-brain is quite extensive as is apparent from an examination of figure 3. Its floor is smooth and exhibits a thickening
at the cephalic end. Caudally there is a flexure of the floor
between the mid-brain and the hind-brain. The floor of the
mesencephalon is thickened at its cephalic end. The trigeminal
ganglion is present as a distinct mass of cells. Its position is
represented in figure 3 by a broken circle. The hind-brain passes
gradually into the spinal cord. A distinct neck flexure is not
present.
The medullary tube has its greatest diameter at the cephalic
extremity. It diminishes gradually in size caudally. At the
caudal neuropore it exhibits a slight enlargement.
Digestive System
The stomodeum is a broad and deep invagination of the ectoderm between the heart bulging and the head. It touches the entoderm of the pharynx and forms with it the beginning of an oral plate. There is no indication of an hypophysis. The ectoderm lining the stomodeum is thickened especially in the roof.
The cephalic extremity of the pharynx projects beyond the oral
plate and nearly reaches the floor of the forebrain, a small
amount of mesoderm intervening.
The median thyreoid anlage is a very prominent evagination
of the entoderm of the floor of the pharynx. It projects between
the layers of splanchnic mesoderm at the arterial end of the
heart immediately caudad to the endothelial aorta and the first
aortic arches. The cephalic wall of the evagination is considerably thicker than the caudal. Cephalad to the thyreoid anlage
the first branchial pouches are evaginated from the lateral wall
of the pharynx and immediately caudad to the thyreoid the
second pair of pouches are present. The first pair of pouches
are the larger. Their long axes are directed laterally, cephalad.
and slightly dorsal. Opposite the venous opening of the heart
the liver anlage is present as a thickening of the gut entoderm.
Lung buds have not developed in this stage.
A cross section of the foregut has a crescentic outline with the
concavity directed dorsally. The tube is widest at the point
where the first pair of branchial pouches is developed. The
cephalic part of the foregut is flattened dorso-ventrally. Caudally the dorso-ventral diameter increases gradually to the
end of the foregut where it becomes greater than the lateral
diamter.
The gut entoderm extending out into the yolk stalk retains its thickness only a short distance (fig. 3).
The hind-gut is shorter than the foregut. Its dorso-ventral
diameter is comparatively large while its lateral diameter is small.
The allantois is evaginated from the ventral wall. The lumen
of the diverticulum is very small at its proximal end, but throughout the rest of its extent it is distinct. At first the allantois
lies between the allantoic arteries. At its distal end it comes
to lie between the venous and arterial trunks or sinuses of the
belly stalk. The end of the allantois is not recurved as found
by Lewis (’12) but ends as a straight tube. The hind-gut exhibits a dorso-ventral constriction immediately cephalad to the
allantoic diverticulum. Caudal to the allantois the hind-gut widens out to form the cloaca. The entoderm of the ventral wall
Cite this page: Hill, M.A. (2024, April 19) Embryology Paper - A Human Embryo of Thirteen Somites. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_A_Human_Embryo_of_Thirteen_Somites
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
