Photograph 1 of 37: 3-4 mm Frog Tadpole
Section 8 of 336 through anterior tip of prosencephalon
The outer ring of darkly staining tissue is the presumptive epidermis (skin ectoderm) of the head
region of the embryo. The inner ring of darkly staining tissue is the wall of the prosencephalon or
forebrain (the most anterior portion of the neural tube). The cavity in the center of the
prosencephalon (neural tube) is the neurocoel.
Photograph 2 of 37: 3-4 mm Frog Tadpole
Section 18 of 336 through prosencephalon and olfactory placodes
Progressing caudally, the prosencephalon appears larger than in the previous photograph
(section 8/336, photo 1/37). Lateral to the ventral portion of the prosencephalon, two darkly
staining structures are visible. These are the olfactory placodes. The spaces between the
presumptive epidermis (skin ectoderm) and the walls of the prosencephalon are filled with moderately
staining head mesenchyme (mesoderm with additional cells of neural crest origin).
At this embryonic stage, a number of organ rudiments are being developed in the head region of the
embryo. In addition to the brain, three paired organs of special sense are also developing from
ectoderm. The most anterior of these sense organ rudiments are the olfactory placodes (smell),
followed by the optic cups and lens vesicles (sight), and most posterior are the otic capsules (sound).
Memory tip: Remember the anterior to posterior order by the organs on your own head, that is, nose,
eye, ear or by the senses involved, that is, smell, sight, sound.
Photograph 3 of 37: 3-4 mm Frog Tadpole
Section 22 of 336 through prosencephalon, olfactory placodes, and optic cup
In this more caudal section, the prosencephalon and olfactory placodes are still visible. Above the
right olfactory placode, another darkly staining structure is visible. This is a portion of the right
optic cup. On the ventral surface of the embryo, an indentation in the midline of the embryo is seen.
This is the roof of the stomodeum, which is an inward growing pocket (an invagination) of ectoderm.
Photograph 4 of 37: 3-4 mm Frog Tadpole
Section 27 of 336 through prosencephalon, optic cups, and stomodeum
The height of the cross section has increased and the prosencephalon occupies a position in the
upper (dorsal) half of the section rather than a central position. The optic cups appear on both
sides of the prosencephalon. On the right side the lens vesicle is seen partially surrounded by the
convex surface of the optic cup. Below (ventral to) the prosencephalon a small cavity surrounded by
ectoderm is visible. This is the cavity of the stomodeum and it is continuous with the exterior of
the embryo. On the right side of the ventral surface of the embryo, a portion of an adhesive gland
(also called mucous gland, cement gland or oral sucker) is visible.
Photograph 5 of 37: 3-4 mm Frog Tadpole
Section 32 of 336 through mesencephalon, prosencephalon,optic cups, oral plate, and adhesive gland
In this section, portions of both the mesencephalon (midbrain) and the prosencephalon
(forebrain) are visible. Because the brain region of the neural tube is not straight but
is bent ventrally, the mesencephalon occupies a more dorsal position and the prosencephalon
occupies a more ventral position with a constriction marking the division between the two.
On each side of the prosencephalon, the optic cups are visible. Note the connection of the
left optic cup to the prosencephalon by a short stalk of tissue that is called the optic stalk.
The optic cups developed from the prosencephalon as paired lateral vesicles (evaginations of the
lateral walls of the prosencephalon). The optic cups will become the retinas of the eyes and the
connecting optic stalks will become the optic nerves that will connect the eyes with the brain.
Ventral to the prosencephalon, the cavity of the stomodeum (as seen in section 27/336, photo 4/37)
is no longer visible. In the same position is a plate of solid tissue. This plate of tissue is
called the oral plate and is composed of a layer of ectoderm (end of stomodeum) meeting with a
layer of endoderm of the foregut. At a later stage, the oral plate will develop a perforation
and the oral cavity (mouth) of the embryo will be open. Memory tip: When ectoderm meets endoderm
without mesoderm between them a perforation usually develops (mouth, cloacal opening, pharyngeal slits).
On the ventral surface, portions of both of the paired adhesive glands are visible. The adhesive
glands will produce a sticky mucus that will allow the attachment of the embryo to a surface after
hatching.
Photograph 6 of 37: 3-4 mm Frog Tadpole
Section 34 of 336 through mesencephalon, infundibulum, optic cups, lens, oral evagination, and adhesive gland
In this section, the mesencephalon appears dorsal to a portion of the prosencephalon called the
infundibulum. The infundibulum develops as a depression in the floor of the prosencephalon and
is the precursor to the posterior portion of the pituitary gland. Ventral to the infundibulum a
small knot of tissue, the hypophysis , is visible. The hypophysis develops as an inward growing
cord of ectoderm from the stomodeum. The hypophysis will form the anterior portion of the pituitary
gland. Lateral to the infundibulum, the optic cups are visible with the lens vesicle present on the
left side. Ventral to the infundibulum and the hypophysis, a cavity, the oral evagination of the
foregut is visible. The walls surrounding the cavity are composed of endoderm.
Photograph 7 of 37: 3-4 mm Frog Tadpole
Section 35 of 336 through mesencephalon, infundibulum, optic cups, lens, oral evagination, and adhesive gland
Section immediately posterior (caudal) to the previous section (section 34/336, photo 6/37).
See section 34 of 336 for a description.
Photograph 8 of 37: 3-4 mm Frog Tadpole
Section 39 of 336 through rhombencephalon, infundibulum, hypophysis, pharynx, and adhesive gland
In the dorsal part of the section the rhombencephalon (hindbrain) is now visible.
The rhombencephalon can be distinguished from the mesencephalon in cross section by the presence
of a thin roof made up of a single layer of flattened, ectodermal cells. Ventral to the
rhombencephalon, the infundibulum and the hypophysis are still visible. Lateral to the infundibulum
on the left side, a portion of the left optic cup can be observed. Dorsal to the position of the
optic cups, a pair of small, darkly staining structures are visible. These structures are a pair
of developing cranial nerves. Nerves outside of the brain and spinal cord are derived from neural
crest. Ventral to the infundibulum and hypophysis, the pharynx (an expanded portion of the foregut)
is visible. On the ventral surface of the embryo, the adhesive glands are easily identified.
Photograph 9 of 37: 3-4 mm Frog Tadpole
Section 42 of 336 through rhombencephalon, infundibulum, hypophysis, pharynx, and adhesive gland
This section is very similar to the previous section (section 39/336, photo 8/37).
See section 39 of 336 for a description.
Photograph 10 of 37: 3-4 mm Frog Tadpole
Section 47 of 336 through rhombencephalon, notochord, thyroid anlage, and adhesive gland
This section is caudal to all portions of the prosencephalon and mesencephalon.
Below the rhombencephalon, the notochord appears as a pale staining oval structure.
Lateral to the notochord on the right side the anterior wall of the otic capsule is visible.
In the ventral half of the section , the pharynx is visible with darkly staining walls and a
large central cavity. Note that the floor of the pharynx has a prominent groove. This groove,
diverticulum, or evagination of the pharynx is the thyroid anlage (rudiment). The darkly staining
tissue located between the skin ectoderm and the lateral walls of pharynx is condensed mesenchyme
(mesoderm) of the second pharyngeal arch. Portions of the adhesive glands are visble on the ventral
surface of the embryo.
Photograph 11 of 37: 3-4 mm Frog Tadpole
Section 49 of 336 through rhombencephalon, notochord, otic capsule, and thyroid anlage
This section is similar to the previous section (section 47/336, photo 10/37).
The right otic capsule appears as are more distinct structure, having a darkly staining
circular wall with a central cavity.
Photograph 12 of 37: 3-4 mm Frog Tadpole
Section 55 of 336 through rhombencephalon, notochord, otic capsules, pharynx, and heart forming region
In this section both otic capsules are seen lateral to the notochord and ventral to the rhombencephalon.
Below the notochord and otic capsules, the pharynx is evident with only a slight groove visible in its floor.
Photograph 13 of 37: 3-4 mm Frog Tadpole
Section 57 of 336 through rhombencephalon, otic capsules, pharynx, bilateral pericardial coelom, and adhesive gland
This section is similar to the previous section (section 55/336, photo 12/37) with respect to the the
appearance of the rhombencepahlon, notochord, otic capsules, and pharynx. The mesodermal region
ventral to the pharynx is becoming a new area of interest because sectioning has reached the heart
forming region. In this section the beginning of the heart forming region is marked by the presence
of two small, thin walled cavities that together are identified as the bilateral, pericardial coelom.
Photograph 14 of 37: 3-4 mm Frog Tadpole
Section 62 of 336 through rhombencephalon, otic capsules, pharynx, anterior portion of heart, and adhesive gland
The rhombencephalon, notochord, and the otic capsule on the left are visible. The pharynx occupies
a position slightly below the center of the section. Ventral to the pharynx, the anterior portion of
the heart can be seen suspended within the pericardial coelom. This section is almost past the
adhesive glands; a small portion of an adhesive gland is visible on the left.
Photograph 15 of 37: 3-4 mm Frog Tadpole
Section 64 of 336 through rhombencephalon, notochord, otic capsules, pharynx, and heart tube
In this section the rhombencephalon and notochord appear relatively unchanged. Lateral and adjacent
to the notochord two, small darkly staining structures are visible. As you follow the sections
caudally, you will be able to determine that small structures are the beginning of the first pair
of somites. Note that in order to simplify the thumbnail diagram only a few of the somites are shown
in the more caudal regions. A small part of the otic capsule on the left is still evident. Ventral
to the pharynx, the developing heart is evident within the pericardial coelom. The coelom is a body
cavity that forms by the delamination (splitting) of the lateral plate mesoderm of the embryo.
The outer layer of mesoderm, next to the ectoderm, is called the somatic mesoderm. The inner layer
of mesoderm, closest to the endoderm, is called the splanchnic mesoderm. It is from splanchnic
mesoderm that the heart and blood vessels of the circulatory system arise.
Photograph 16 of 37: 3-4 mm Frog Tadpole
Section 66 of 336 through rhombencephalon, notochord, pharynx, and heart
Observe the rhombencephalon and notochord. On both sides of the notochord, the anterior portions of
the first pair of somites are visible. The somites are blocks of dorsal, paraxial mesoderm (=epimere)
that will develop into cartilage and bone of the vertebrae, skeletal muscle, and contribute cells to
the dermis of the skin. Ventral to the pharynx, the heart can be observed suspended within the
pericardial coelom by a short strip of splanchnic mesoderm called the dorsal mesocardium. In the
following section (section 68/336, photo 17/37) the parts of the developing heart will be identified.
Photograph 17 of 37: 3-4 mm Frog Tadpole
Section 68 of 336 through rhombencephalon, notochord, pharynx, and heart
Note the rhombencephalon, notochord, somites, and pharynx. Below the pharynx the heart appears
suspended within the pericardial coelom by the dorsal mesocardium. The pericardial coelom is
delimited by a thin layer of somatic mesoderm called the pericardium. The heart is composed of
two layers, an inner layer called the endocardium, and an outer layer called the epimyocardium.
The endocardium will form the lining of the heart wall and the epimyocardium will form the muscle.
Photograph 18 of 37: 3-4 mm Frog Tadpole
Section 71 of 336 through rhombencephalon, notochord, pharynx, posterior portion of heart, and somites
Observe the rhombencephalon, notochord, and somites. Note that the width of the pharyngeal cavity
is beginning to narrow. At this stage, the developing heart tube ends in a mass of undifferentiated
splanchnic mesoderm. This can be observed in this section through the posterior portion of the heart
tube. A diameter of the heart tube is smaller and is merging on its ventral surface with a mass of
mesoderm.
Photograph 19 of 37: 3-4 mm Frog Tadpole
Section 72 of 336 through rhombencephalon, notochord, pharynx, and posterior portion of heart
Section immediately caudal to previous section (section 71/336, photo 18/37). See section 71 of 336
for a description.
Photograph 20 of 37: 3-4 mm Frog Tadpole
Section 78 of 336 through rhombencephalon/spinal cord, notochord, pharynx, and posterior portion of heart forming region
The level of this section is through a region where the rhombencephalon is tapering into the
spinal cord. Note that the thin, walled roof of the rhombencephalon has become narrower.
Identify the notochord, somites, and pharynx. Ventral to the pharynx, is a mass of undifferentiated,
splanchnic mesoderm which marks the posterior portion of the heart forming region.
Photograph 21 of 37: 3-4 mm Frog Tadpole
Section 84 of 336 through spinal cord, notochord, somites, pharynx, and liver diverticulum
Observe the spinal cord and notochord with adjacent somites. Examine the outside shape of the embryo.
Note the paired bulges located just below the horizontal level of the notochord. These paired bulges
of tissue are called the intermediate mesoderm,mesomeres, or nephrotomes. At this stage in development,
the nephrotomes have begun to develop pronephric tubules and collectively may be called the
pronephros or pronephric kidney. Compare the pharynx of this section with the previous section.
The floor of the pharynx has an extremely deep groove. This is the liver diverticulum, the rudiment
of the liver.
Photograph 22 of 37: 3-4 mm Frog Tadpole
Section 89 of 336 through spinal cord, notochord, somites, pronephros, pharynx, and liver diverticulum
Examine the spinal cord and notochord with adjacent somites. In this section, the pronephric
tubules are clearly evident as small, circular profiles within the region of the nephrotomes.
The coelom (body cavity) is also readily identifiable in a position adjacent and medial to the
pronephric tubules. The thin layer of somatic mesoderm can be seen delimiting the outer wall of the
coleom. The inner wall of the coelom is composed of a thin layer of splanchnic mesoderm that is
closely applied to the adjacent, underlying endoderm of the pharynx and is therefore not easily
distinquished. Note that the endodermal tissue surrounding the base of the liver diverticulum has
become thicker than in the previous section (section 84/336, photo 21/37).
Photograph 23 of 37: 3-4 mm Frog Tadpole
Section 94 of 336 through spinal cord, notochord, somites, pronephros, midgut coelom, and yolky endoderm
This section is similar to the previous section (section 89/336, photo 22/37).
Note the spinal cord, notochord, somites, pronephros, and coelomic cavities. This section is at a
level caudal to the liver diverticulum. In this region the foregut (pharynx) narrows into the midgut.
Note the presence of an increased mass of yolky endoderm. The yolky endoderm is the nutrient supply
for the developing embryo.
Photograph 24 of 37: 3-4 mm Frog Tadpole
Section 101 of 336 through spinal cord, notochord, somites, pronephros, coelom, midgut coelom, and yolky endoderm
This section is similar to the previous section (section 94/336, photo 23/37).
Fewer pronephric tubules are visible within the pronephros and the width of the embryo is increased
with the presence of increased yolky endoderm.
Photograph 25 of 37: 3-4 mm Frog Tadpole
Section 106 of 336 through spinal cord, notochord, somites, pronephros, coelom, midgut, and yolky endoderm
The spinal cord gradually tapers off in diameter from the rhombencephalon to its caudal end.
Note that in this section the diameter of the spinal cord approximates that of the notochord.
Wedged between the notochord and the midgut, a small knot of cells can be identified that are part
of the subnotochordal rod. The subnotochordal rod is a transient structure of endodermal origin and
unknown function that will later disappear.
Photograph 26 of 37: 3-4 mm Frog Tadpole
Section 112 of 336 through spinal cord, notochord, somites, coelom, subnotochordal rod, midgut, and yolky endoderm
Examine sections 112 to 187 (photos 26 to 32). Note the structural continuity of the spinal cord,
notochord, subnotochordal rod, coelomic cavities, midgut, and mass of yolky endoderm.
As you proceed through the sections, the subnotochordal rod and the coelomic cavities will gradually
become less prominent. The dorsal surface of the embryo becomes raised upward as the dorsal fin.
Photograph 27 of 37: 3-4 mm Frog Tadpole
Section 119 of 336 through spinal cord, somites, notochord, subnotochordal rod, midgut, and yolky endoderm
Photograph 28 of 37: 3-4 mm Frog Tadpole
Section 129 of 336 through spinal cord, notochord, somites, midgut, and yolky endoderm
Photograph 29 of 37: 3-4 mm Frog Tadpole
Section 141 of 336 through spinal cord, notochord, somites, midgut, yolky endoderm, and dorsal fin
Photograph 30 of 37: 3-4 mm Frog Tadpole
Section 151 of 336 through spinal cord, notochord, somites, midgut, yolky endoderm, and dorsal fin
Photograph 31 of 37: 3-4 mm Frog Tadpole
Section 166 of 336 through spinal cord, notochord, somites, midgut, yolky endoderm, and dorsal fin
Photograph 32 of 37: 3-4 mm Frog Tadpole
Section 187 of 336 through spinal cord, notochord, somites, midgut, yolky endoderm, and dorsal fin
Photograph 33 of 37: 3-4 mm Frog Tadpole
Section 202 of 336 through spinal cord, notochord, midgut/hindgut, yolky endoderm, and dorsal fin
This section is at the transition zone between the midgut and the hindgut. Note that the height and
width of the section has decreased with a decrease in the bulk of the yolky endoderm.
Photograph 34 of 37: 3-4 mm Frog Tadpole
Section 217 of 336 through spinal cord, notochord, hindgut, and somites
This section is at the level of the hindgut. Note that the hindgut occupies a position in the
ventral half of the section and that the mass of yolky endoderm is greatly reduced.
Photograph 35 of 37: 3-4 mm Frog Tadpole
Section 231 of 336 through spinal cord, notochord, somites, and proctodeum
This section through the base of the tail, is at the level of the proctodeum. The proctodeum is
an ectodermal invagination that meets with the endoderm of the hindgut. Note the delicate strand of
tissue between the proctodeum and hindgut. This is the cloacal membrane (ectodemal/endodermal plate)
that will become perforated to form the posterior opening of the digestive tract.
Photograph 36 of 37: 3-4 mm Frog Tadpole
Section 271 of 336 through tail, dorsal and ventral fins
In this section through the tail, the dorsal and ventral fins are clearly evident. Note the decrease
in the diameters of the spinal cord, notochord, and adjacent somites.
Photograph 37 of 37: 3-4 mm Frog Tadpole
Section 296 of 336 through tail, dorsal and ventral fins
A section through the caudal half of the tail but not yet at the tip. Forty more sections were
required to reach
THE END OF THIS TADPOLE.