Photograph 1 of 44: 5-7 mm Frog Tadpole
Section 10 of 400 through anterior tip of prosencephalon and olfactory pit
The outer ring of dark staining tissue is the presumptive epidermis (skin ectoderm) of the head region of
the embryo. The round indentation seen on the right hand side of the embryo is an olfactory pit. The median
cavity surrounded by a ring of darkly staining tissue is the anterior tip of the prosencephalon. Moderately
staining head mesenchyme is visible between the neural ectoderm of the prosencephalon and the outer skin
ectoderm.
Photograph 2 of 44: 5-7 mm Frog Tadpole
Section 15 of 400 through prosencephalon, olfactory pits, and oral cavity
Progressing caudally, the prosencephalon appears larger than in the previous photograph (section 10/400,
photo 1/44). Lateral to the ventral portion of the prosencephalon, both olfactory pits are now visible. The
olfactory pits began as two thickenings (placodes) of ectoderm (recall the olfactory placodes of 3-4 mm frog)
that invaginated and lengthened to form pits. The olfactory pits will subsequently develop into the nasal
passages with olfactory receptors. The large indentation on the ventral surface of the embryo marks the
beginning of the oral cavity. At this stage in development the oral plate has become perforated and the
embryo now has an open mouth.
Photograph 3 of 44: 5-7 mm Frog Tadpole
Section 20 of 400 through prosencephalon, olfactory pit, and oral cavity
The prosencephalon and olfactory pits can still be seen in this section. The oral cavity is the large
indentation on the ventral surface of the embryo. Note the developing lateral tooth protruding into the
oral cavity on the right hand side. The cell layer that comprises the epithelial roof of the oral cavity
has separated from the overlying mesenchyme to create an artificial space which was not present in the living
embryo. (Any space, structure, or other feature that is not typical of the actual specimen, but is created by
post mortem changes, fixation, or histological manipulations is called an artifact.) This artficial space
(artifact) continues through the anterior portions of the pharynx and ends at the region where the notochord
begins.
Photograph 4 of 44: 5-7 mm Frog Tadpole
Section 25 of 400 through prosencephalon, optic cup, oral cavity, and adhesive gland
This section is caudal to the olfactory pits. The prosencephalon and the oral cavity are visible. On the
right hand side, a portion of the optic cup and an adhesive gland can be observed.
Photograph 5 of 44: 5-7 mm Frog Tadpole
Section 30 of 400 through prosencephalon, optic cups, lens, oral cavity, and adhesive glands
Lateral to the prosencephalon, the optic cups are evident. On the right hand side, the lens (lens vesicle) is also
visible. The optic cups developed as paired, lateral evaginations of the prosencephalon which subsequently caused
the induction of the paired lens vesicles from the overlying ectoderm. Lateral and ventral to the oral cavity,
darkly staining knots of condensed mesenchyme are present. These will develop into the jaw cartilages.
Photograph 6 of 44: 5-7 mm Frog Tadpole
Section 35 of 400 through mesencephalic-prosencephalic constriction, optic cups, oral cavity, and adhesive glands
Portions of both the mesencephalon and prosencephalon are evident in this section with a constriction between them. The optic
cups are seen lateral to the brain. The oral cavity is visible surrounded by presumptive jaw cartilages. Adhesive glands are
present on the ventral surface of the embryo.
Photograph 7 of 44: 5-7 mm Frog Tadpole
Section 40 of 400 through mesencephalon, infundibulum, optic cup, oral cavity/pharynx, and adhesive glands
A portion of the prosencephalon, the infundibulum, is seen below the mesencephalon. The infundibulum will form the posterior
part of the pituitary gland. The hypophysis, an outgrowth of the stomodeal invagination, loses its original connection with
the stomodeum and grows around the infundibulum to form the anterior part of the pituitary gland. At this stage in development,
the hypophysis is no longer visible as a distinct cord of cells as it was seen at the 3-4 mm stage.
Photograph 8 of 44: 5-7 mm Frog Tadpole
Section 45 of 400 through mesencephalon, wall of infundibulum, and pharynx
Below the mesencephalon, a small portion of the wall of the infundibulum is evident. Lateral to the floor of the
mesencephalon on the right hand side, a developing cranial nerve ganglion can be observed. The developing cranial nerve
ganglion is part of the peripheral nervous system and is derived from neural crest cells. On the left hand side, the posterior
portion of the optic cup is still visible. Note the shape of the pharynx. In the 3-4 mm frog the pharynx had an expanded
cavity with height and width being approximately equal. In the 5-7 mm frog the width of the pharyngeal cavity greatly exceeds
the height.
Photograph 9 of 44: 5-7 mm Frog Tadpole
Section 50 of 400 through rhombencephalon, otic capsule, notochord, pharynx, pericardial coelom, and truncus arteriosus
The sectioning level is now through the hindbrain (rhombencephalon). Note the characteristic roof of the rhombencephalon composed of a single
layer of flattened cells. The notochord is now visible ventral to the rhombencephalon. Beneath the pharynx, the most anterior
portion of the heart, the truncus arteriosus is evident within the pericardial coelom. The heart has four subdivisions which
can be observed in the following four sections. These subdivisions in anterior to posterior order are: the truncus arteriosus,
the ventricle, the atrium, and the sinus venosus. Remember that the blood flows through the embryonic heart from the posterior
(sinus venosus) to the anterior (truncus arteriosus).
Photograph 10 of 44: 5-7 mm Frog Tadpole
Section 55 of 400 through rhombencephalon, otic capsule, notochord, pharynx, pharyngeal cleft, pericardial coelom, truncus arteriosus, and ventricle
Note the rhombencephalon, the otic capsule on the right, the notochord and the pharynx. On the right hand side of the pharynx,
a pharyngeal cleft can be seen. Within the pericardial coelom, two regions of the heart can be observed. The truncus
arteriosus is evident on the left and the ventricle is evident on the right.
Photograph 11 of 44: 5-7 mm Frog Tadpole
Section 60 of 400 through rhombencephalon, otic capsules, notochord, pharynx, pharyngeal clefts, external gills, pericardial coelom, atrium, and ventricle
Both otic capsules are now visible within the section. Recall that the otic capsules developed from ectoderm and will later
differentiate into the inner ears. Sectioning of the external gills is evident on the outer surface of the pharyngeal region
(right side).. The external gills are finger-like projections that protrude from the sides of the head. Only small portions
of the external gills are visible in any given section. In the heart forming region, the ventricle is still seen with the
atrium appearing above.
Photograph 12 of 44: 5-7 mm Frog Tadpole
Section 65 of 400 through rhombencephalon, otic capsules, notochord, pharynx, pharyngeal clefts, external gills, trachea from laryngotracheal groove, pericardial coelom, atrium, and liver
A new tubular structure of interest, the trachea, is evident below the pharynx. The trachea developed from endoderm of
the floor of the pharynx. In this region of the pharynx, the floor developed a groove (the laryngotracheal groove) that
deepened and became pinched off from the pharynx forming a separate tube. It will be seen in the next section (section 70/400,
photo 13/44) that the trachea bifurcates into the lung buds (future bronchi and lungs of the air breathing, metamorphosed frog). In the heart forming region, the atrium is visible. Below the atrium and to the right is the anterior portion of the liver.
Photograph 13 of 44: 5-7 mm Frog Tadpole
Section 70 of 400 through rhombencephalon, otic capsule, notochord, somite, pharynx/esophagus, external gills, lung buds, coelom, sinus venosus, and liver
On both sides of the notochord, the anterior portions of the first pair of
somites are evident. Ventral to the notochord, the pharynx has narrowed indicating
the zone of transition from pharynx to esophagus. The lumen of the pharynx/esophagus
is not visible making this region of the gut tube appear as a solid structure.
Immediately below the pharynx/esophagus note the bifurcation of the trachea into
the two lung buds. The tubular lung buds extend caudally.
Below the lung buds, the most posterior portion of the developing heart, the sinus venosus, is visible. Below the sinus
venosus the liver is visible. Recall that in the 3-4 mm frog, the first visible rudiment of the the liver was as the
diverticulum of the pharynx. The liver in the 5-7 mm frog embryo is beginning to show the characteristic arrangement of liver
cells into anastomosing, epithelial cords.
Photograph 14 of 44: 5-7 mm Frog Tadpole
Section 75 of 400 through rhombencephalon, notochord, somites, pronephric tubules, esophagus, lung buds, external gills, coelom, liver, and gall bladder
At this level the lung buds appear as tubules surrounded by a small amount of tissue that actually project into the coelomic
cavity. On the right hand side the pronephric tubules of the kidney are seen. Near the ventral surface of the liver the
rudiment of the gall bladder is also visible.
Photograph 15 of 44: 5-7 mm Frog Tadpole
Section 80 of 400 through rhombencephalon, notochord, dorsal aorta, somites, pronephric tubules, coelom, lung buds, coiled gut, and liver
In this section the notochord is not found in the midline but is displaced to the right. This is an artifact that probably
occurred at the time of fixation or embedding of the specimen. Ventral to the expected position of the notochord is a blood
vessel called the dorsal aorta. The dorsal aorta, formed from the fusion of a pair of blood vessels (plural: dorsal aortae),
carries oxygenated blood from the gills. The lung buds are still visible at this level of sectioning. On the right side, the
caudal tip of the lung bud has been reached. In the ventral half of the embryo, note the presence of the beginning of the
coiled gut.
Photograph 16 of 44: 5-7 mm Frog Tadpole
Section 85 of 400 through rhombencephalon/spinal cord, notochord, dorsal aorta, somites, pronephric tubules, coelom, glomi, coiled gut, and yolky endoderm
Note that the rhombencephalon is decreasing in height and width and that the thin-walled roof is also decreasing in width.
The rhombencephalon gradually tapers into the spinal cord. Locate the notochord, the adjacent somites, the dorsal aorta, and
the pronephric tubules.
In this section, two triangular-shaped structures are seen ventral to the dorsal aorta that hang down into the coelomic cavity.
These structures are glomi (singular: glomus) which are tufts of small blood vessels surrounded on their lateral and ventral
surfaces by the thin wall of the coelom. The glomi are functional components of the pronephric kidney. Waste products from
the blood diffuse from the glomi into the coelomic fluid. The coelomic fluid is swept through ciliated funnel-shaped openings
(nephrostomes) into the pronephric tubules where it can be processed. Nitrogenous wastes are passed from the pronephric tubules
into the pronephric ducts, from the pronephric ducts to the hindgut (cloaca), and finally to the exterior.
The coiled nature of the developing gut is readily evident in this section. The frog tadpole will become a filter feeder and
needs a very long intestine.
Photograph 17 of 44: 5-7 mm Frog Tadpole
Section 90 of 400 through rhombencephalon/spinal cord, notochord, dorsal aorta, somites, pronephric tubules, coelom, glomi, coiled gut, and yolky endoderm
Section similar in appearance to the previous section. See section 85/400, photo 16/44 for a
description.
Photograph 18 of 44: 5-7 mm Frog Tadpole
Section 95 of 400 through spinal cord, notochord, dorsal aorta, somites, pronephric tubules, coelom, coiled gut, and yolky endoderm
Observe the spinal cord, notochord, and paired somites. The posterior portions of the pronephric
kidney are still visible at this level. On the left side, observe the opening of the distal end of
the pronephric tubule into the pronephric duct (lower tubule). Notice that the coiled portion of the
developing intestine ends in a mass of yolky endoderm.
Photograph 19 of 44: 5-7 mm Frog Tadpole
Section 100 of 400 through spinal cord, notochord, dorsal aortae, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
The paired dorsal aortae are evident in this region of the embryo. At this level of sectioning, the
pronephric tubules can no longer be seen but the paired pronephric ducts
can be seen. While the pronephric tubules are convoluted structures, the pronephric ducts are
relatively straight. The pronephric ducts can be followed caudally through the sections until they
open into the hindgut (section 165/400, photo 32/44).
Photograph 20 of 44: 5-7 mm Frog Tadpole
Section 105 of 400 through spinal cord, notochord, dorsal aortae, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
The dorsal mesentery is clearly evident in this section and appears as a short stalk of tissue
attaching the mass of yolky endoderm to the dorsal body wall. The dorsal mesentery consists of a
double layer of splanchnic mesoderm and extends from the esophagus to the hindgut.
Photograph 21 of 44: 5-7 mm Frog Tadpole
Section 110 of 400 through spinal cord, notochord, dorsal aortae, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Trace the splanchnic mesoderm from one side of the dorsal mesentery around the yolky endoderm to the
other side of the dorsal mesentery. Note the many blood vessels that have developed within the
splanchnic mesoderm in this region.
Photograph 22 of 44: 5-7 mm Frog Tadpole
Section 115 of 400 through spinal cord, notochord, dorsal aorta, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Note that the dorsal aortae have again fused into a single blood vessel.
In the following sections (Sections 120 to 165, photos 24 to 32), observe the structural
continuity of the spinal cord, notochord, dorsal aorta, pronephric ducts, coelom, and dorsal
mesentery. Notice that the yolky endoderm will gradually diminish as sectioning approaches the
hindgut (Section 165/400, photo 32/44). Note the raised ridge of tissue on the dorsal surface of the
embryo. This is the dorsal fin of the tadpole.
Photograph 23 of 44: 5-7 mm Frog Tadpole
Section 120 of 400 through spinal cord, notochord, dorsal aorta, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Photograph 24 of 44: 5-7 mm Frog Tadpole
Section 125 of 400 through spinal cord, notochord, dorsal aorta, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Photograph 25 of 44: 5-7 mm Frog Tadpole
Section 130 of 400 through spinal cord, notochord, dorsal aorta, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Photograph 26 of 44: 5-7 mm Frog Tadpole
Section 135 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Photograph 27 of 44: 5-7 mm Frog Tadpole
Section 140 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Photograph 28 of 44: 5-7 mm Frog Tadpole
Section 145 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Photograph 29 of 44: 5-7 mm Frog Tadpole
Section 150 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Photograph 30 of 44: 5-7 mm Frog Tadpole
Section 155 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Photograph 31 of 44: 5-7 mm Frog Tadpole
Section 160 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, coelom, dorsal mesentery, and yolky endoderm
Photograph 32 of 44: 5-7 mm Frog Tadpole
Section 165 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, and hindgut
Note the small size of the spinal cord. Compare its diameter with that of the notochord.
The hindgut (cloaca) is visible in this section. The pronephric ducts empty into the hindgut at this
level. Examine the hindgut through the following sections (Sections 170 to 190, photos 33 to 37)
until the opening to the exterior is seen.
Photograph 33 of 44: 5-7 mm Frog Tadpole
Section 170 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, and hindgut
Photograph 34 of 44: 5-7 mm Frog Tadpole
Section 175 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, and hindgut
Photograph 35 of 44: 5-7 mm Frog Tadpole
Section 180 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, and hindgut
Photograph 36 of 44: 5-7 mm Frog Tadpole
Section 185 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, pronephric ducts, and hindgut
Photograph 37 of 44: 5-7 mm Frog Tadpole
Section 190 of 400 through dorsal fin, spinal cord, notochord, dorsal aorta, somites, and proctodeum
Note the ending of the gut at the proctodeum. In the following sections (Sections 195 to 370,
photos 38 to 44), examine the structures of the tail.
Photograph 38 of 44: 5-7 mm Frog Tadpole
Section 195 of 400 through tail, dorsal fin, spinal cord, notochord, dorsal aorta, somites, and ventral fin
Photograph 39 of 44: 5-7 mm Frog Tadpole
Section 220 of 400 through tail, dorsal fin, spinal cord, notochord, dorsal aorta, somites, and ventral fin
Photograph 40 of 44: 5-7 mm Frog Tadpole
Section 250 of 400 through tail, dorsal fin, spinal cord, notochord, dorsal aorta, somites, and ventral fin
Photograph 41 of 44: 5-7 mm Frog Tadpole
Section 280 of 400 through tail, dorsal fin, spinal cord, notochord, somites, and ventral fin
Photograph 42 of 44: 5-7 mm Frog Tadpole
Section 310 of 400 through tail, dorsal fin, spinal cord, notochord, somites, and ventral fin
Photograph 43 of 44: 5-7 mm Frog Tadpole
Section 340 of 400 through tail, dorsal fin, spinal cord, notochord, somites, and ventral fin
Photograph 44 of 44: 5-7 mm Frog Tadpole
Section 370 of 400 through tail, dorsal and ventral fins
The spinal cord and notochord are no longer visible in this posterior portion of the tail.
Thirty more sections were required to reach
THE END OF THIS TADPOLE