| 1 |
Fertilized egg, without polar bodiesb |
--- |
| 2 |
First maturation division. First polar body formed
|
20 min |
| 3 |
Second maturation division. Second polar body formed; blastodisc formed
|
40 min |
| 4 |
First cleavage: The first cleavage furrow begins at the center of the blastodisc at the animal pole. Cleavage is incomplete; it divides the
blastodisc but not the underlying yolk mass. The first cleavage furrow marks the plane of bilateral symmetry, establishing
the future right and left halves of the embryo.
|
8 h |
| 5 |
Second cleavage: The second cleavage furrow forms obliquely at the midpoint of the first furrow, producing two large and two smaller cells.
The second division divides the left and right halves of the embryo into a larger anterior and a smaller posterior half. By
the second cleavage, the three body axes (anteroposterior, dorsoventral, left-right) are established. The strong contraction
of the second furrow produces a central gap (typical of large cephalopod eggs [von Boletzky 1988a]) between the four daughter
cells, exposing the underlying yolk mass.
|
10 h |
| 6 |
Third cleavage: The angle formed between the third and second cleavage furrows is slightly oblique, and divides the two larger anterior cells
into two equal parts. The posterior region of the cleavage furrow is parallel to the first cleavage plane and results in two
smaller regions adjacent to the first furrow. Through this stage, the embryo is a syncytium.
|
12 h |
| 7 |
Fourth cleavage: This produces the first four blastomeres of the embryo. The other 12 cells remain cytoplasmically connected at their outer
margins, forming the syncytial blastocones.
|
14 h |
| 8 |
Fifth cleavage: 32-cell stage
|
16 h |
| 9 |
Sixth and seventh cleavage: 64- and 128-cell stages, respectively
|
18-20 h |
| 10 |
Formation of the blastoderm: At the end of cleavage, the future embryo is represented by the blastoderm (a single layer of blastomeres surrounded by lines
of blastocones) at the animal pole. The blastoderm gives rise to the embryo proper and the external yolk sac.
|
1 d |
| 11 |
Gastrulation and formation of the germ layers: Blastomeres at the periphery of the blastoderm are covered over by adjacent interior blastomeres. The submerged blastomeres
form a ring of mesendoderm at the periphery of the blastoderm (von Boletzky 1988a,b; Fioroni 1990). The single cell-layered
yolk papilla is visible at the center of the blastoderm.
|
2 d |
| 12 |
Blastoderm expansion: The blastoderm expands over the yolk by division of the outer layer of blastomeres at the lateral periphery. The mesendoderm
spreads toward the center of the blastoderm beneath the outer layer, creating the embryonic germ layers, decreasing the size
of the yolk papilla in the process.
|
3 d |
| 13 |
Blastoderm encompasses 30% of yolk mass: The blastoderm continues to expand through cell divisions and covers ~30% of the surface of the egg. The yolk papilla at
the animal pole continues to decrease in diameter with the development of the inner germ layers.
|
4 d |
| 14 |
Blastoderm covers 40% of yolk mass |
5 d |
| 15 |
Blastoderm covers ~50% of the surface of the egg |
6 d |
| 16 |
Gastrulation nearly complete: The blastoderm covers ~60% of the egg surface.
|
7 d |
| 17 |
Early organogenesis: The blastoderm covers 80%-90% of egg, with only a small plug of yolk at the vegetal pole. The embryo rotates freely within
the chorion by moving the cilia on the external yolk sac (von Boletzky 1971). The eye primordia appear as two bilaterally
positioned oval ectodermal thickenings; the shell gland primordium first becomes visible as a slight oval depression at the
former animal pole.
|
8 d |
| 18 |
Eye placodes and arm band primordia visible; shell gland invaginates; stomodeum appears: The blastoderm completely encloses the external yolk mass. Organ primordia are slightly more apparent. Eye ectodermal placodes
thicken, elevate, and position bilaterally toward the anterior. The shell sac border elevates slightly as the oval shell gland
primordium invaginates. The mantle primordium is first visible as a thickening around the shell gland at the former animal
pole. Arm primordia arise as two thickened bands of tissue located laterally at the “equator” of the embryo. The mouth first
appears as a crescent-shaped placode (stomodeum) between the eye primordia. The funnel complex primordia are present on the
posterior surface as two pairs of bilateral thickened placodes, the dorsal and ventral funnel folds.
|
9 d |
| 19 |
Optic vesicles begin to form; shell gland oval; five discrete arm pairs discernible; mantle thickens; stomodaeum invaginates;
statocyst and gill primordia visible; anal papilla present: The organ primordia are more readily apparent. The eye ectodermal placodes internalize as the annular folds thicken around
the periphery, forming the optic vesicle. The elliptically shaped shell gland continues to invaginate, but remains open. The
mantle primordium thickens and lengthens. Each thickened band of the arm primordia divides into five discrete pairs of arm
buds (numbered I-V from anterior to posterior). The arm buds are more closely spaced at the posterior surface; there is a
large gap between the first pair of anterior arm buds. The stomodeum invaginates. The dorsal and ventral funnel folds thicken.
Statocyst primordia appear as two shallow circular depressions located laterally on the posterior surface between the dorsal
and ventral funnel folds. Paired gill primordia appear as thickened placodes on the posterior surface below the ventral edge
of the mantle primordium. The anal papilla appears at the midline between the gill primordia.
|
10 d |
| 20 |
Optic vesicle 50%-90% complete; fin primordia appear; salivary pit visible; statocysts invaginate; medial edges of dorsal
and ventral funnel folds grow toward midline: Internalization of the eye vesicle continues; the annular fold closes around the periphery of the placode. Shell gland invagination
continues; the edges of the shell sac begin to close. The paired fin primordia appear as thickened crescent-shaped placodes
on the mantle lateral to the outer edges of the shell gland. Each arm bud is more distinct. The stomodeum continues to invaginate;
the salivary pit is visible as a secondary depression within the mouth. The dorsal and ventral funnel folds elevate and the
medial edges of each pair grow toward the midline. Statocyst primordia begin to invaginate. The paired gill primordia and
anal papilla appear as small nubs.
|
11 d |
| 21 |
Optic vesicles complete; retina lightly pigmented; optic lobe primordia visible; shell sac closing; sucker primordia visible;
ventral funnel folds fused at midline: Formation of the eye vesicles is complete. The retinas are pigmented light orange. Paired optic lobe (optic ganglia) primordia
are evident as masses of tissue located posteroventrally to the eye vesicles. The diamond-shaped opening of the shell sac
starts to close, with fusion lines radiating from the four points. The distal edges of the fin primordia begin to separate
from the mantle. The first sucker primordia are evident on the arms. The region surrounding the stomodeum thickens and elevates
along the dorsal and lateral edges. The salivary pit shifts toward the dorsal edge of the stomodeum. The ventral funnel folds
fuse at the midline, forming a continuous fold. The edges surrounding the statocysts thicken as invagination continues.
|
12 d |
| 22 |
Optic lobes prominent; lens primordia visible; shell sac closed; fin edges separate from mantle; mantle covers one-third of
the gills; dorsal and ventral funnel folds fused along length: Pigmentation of the retina increases (light orange). The optic lobes become prominent masses comprising lateral bulges on
either side of the head. The lens primordia are visible within the eye vesicles. The shell sac is completely closed. The fins
have separated from the mantle. The mantle increases in length; the ventral edge covers one-third the length of the gills.
The number of suckers increases. The mouth is still positioned on the anterior surface between arm pair I. The salivary pit
is no longer visible from the anterior surface as the stomodeum grows deeper and the dorsal edge thickens. The dorsal and
ventral funnel folds fuse to each other along their lengths; the funnel complex appears as a “W” when viewed from the posterior.
Statocysts have completely invaginated.
|
13 d |
| 23 |
Retina orange and cup-shaped; mantle covers one-half of the gills; medial edges of dorsal and ventral funnel folds fused to
form funnel tube; external yolk sac separated from embryo; internal yolk sac single-lobed: The retina is cup-shaped and the lens is visible as a clear rod in the center of the eye vesicle. The folds of the iris have
occurred (Arnold 1965; Baeg et al. 1992), but are difficult to see. The fins are rounded and larger. The mantle covers one-half
the length of the gills. The brachial crown contracts, decreasing the distance between arm pair I as they migrate toward each
other, dorsal to the mouth. The medial edges of the dorsal and ventral funnel folds fuse ventrally at the midline, forming
the beginning of the funnel tube. The external yolk sac is well separated from the embryo, and the animal-vegetal axis flexes
such that the posterior surface of the embryo will come to lie near the posterior surface of the external yolk sac. The single
lobe of the internal yolk sac is visible within the mantle from the anterior surface.
|
14 d |
| 24 |
Retina dark orange; mantle covers gills totally; mouth ventral within brachial crown; medial margins of funnel tube fused
50%-90%; branchial heart contractions; organ of Hoyle visible: Pigmentation of the retina increases (dark orange). Fins are well separated from the mantle, which completely covers the
gills and anal papilla; the funnel retractor muscles are still visible under the ventral edge of the mantle. Brachial crown
contraction is complete: Arm pair I meets at the midline, and the mouth is now located ventrally, completely within the brachial
crown. The median margins of the funnel folds continue to fuse from ventral to dorsal; the fusion is 50%-90% complete. The
internal yolk sac appears bilobed. Branchial hearts, located at the base of the gills, start to contract irregularly. The
organ of Hoyle (hatching gland) first appears at the dorsal tip of the mantle.
|
15 d |
| 25 |
Iris clearly visible; mantle covers dorsal edge of funnel (small triangular opening remains); gill filaments visible; primary
lid forming; internal yolk sac bilobed; organ of Hoyle arrow-shaped; external yolk sac longer than body: The iris is visible and pigmented dark orange. The lens is spherical. The fins are separated from the mantle, with only the
median margins still connected. The ventral mantle edge covers the dorsal portion of the funnel, but a small triangular opening
remains. Contractions of the mantle can occur. Gill filaments are visible through the mantle. The statocysts are completely
developed with statoliths. The two lobes of the internal yolk sac increase in size and extend toward the dorsal end of the
mantle. The arrow-shaped organ of Hoyle is prominent and the terminal spine located at its tip is evident. The external yolk
sac remains longer than the body of the embryo. The primary lid of the eye begins to develop as dorsal tissue extensions from
arm pairs III and IV.
|
16 d |
| 26 |
Retina reddish; mantle completely covers dorsal portion of funnel; primary lid nearly complete; internal yolk sac with four
lobes; chromatophores first visible: Pigmentation of the retina deepens from dark orange to red. Pigmentation of the iris also increases. The mantle completely
covers the dorsal margin of the funnel. A second pair of small lobes forms laterally from the internal yolk sac. The primary
lid is nearly complete. A few light orange chromatophores become visible on the mantle and the head.
|
17 d |
| 27 |
Late developmental stages: The eyes can move freely in their orbits. The second pair of lobes of the internal yolk sac increases in size. Dorsal chromatophores
become evident and the total number of chromatophores increases.
|
18 d |
| 28 |
Primary lid complete; iridophores present on eye; external yolk sac about size of head; olfactory tubercles present: The embryo possesses the characteristics of the adult form, having the appearance of a miniature adult. The primary lid completely
covers the eyes, and pale yellow patches of iridophores (which reflect light) are present on the eye. The external yolk sac
has been depleted further and is comparable in size to the head of the embryo. Olfactory tubercles are present as two bilaterally
positioned circles on the ventral surface of the head, under the optic lobes (Arnold 1965; Baeg et al. 1992). In practice,
these are very difficult to see (they are mostly obscured by the ventral edge of the mantle), but can be clearly distinguished
using tubulin probes.
|
19 d |
| 29 |
Ink sac visible under posterior mantle; external yolk sac nearly depleted: Pigmentation of the ink sac is visible from the posterior surface within the mantle cavity as a black ovoid area at the dorsal-most
end of the funnel tube. The external yolk sac is nearly depleted and is mostly obscured by the arms. Hatching can occur prematurely
if the embryos are disturbed.
|
20 d |
| 30 |
Hatching stage: The small external yolk sac is dropped and the organ of Hoyle becomes depleted upon hatching. The internal yolk sac persists
and is slowly consumed during the first day after hatching.
|
21 d |
|
| aApproximate time post-fertilization at 24°C.
|
| bNot observed.
|
