embryology

# What is an embryo?

embryo (wn, noun)  
: embryo -- ((botany) a minute rudimentary plant contained within a seed or an archegonium)  
: embryo, conceptus, fertilized egg -- (an animal organism in the early stages of growth and differentiation that in higher forms merge into fetal stages but in lower forms terminate in commencement of larval life)

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## Gamete genesis and fertilization

<div><img src="figs/IMG_0184_vertebrate_spermatozoa_4d528be.jpg" width="200px"><figcaption>vertebrate spermatozoa</figcaption></div>

<div><img src="figs/IMG_0185_oogenesis_fertilization_fd15ef0.jpg" width="400px"><figcaption>spermatogenesis, oogenesis, fertilization</figcaption></div>

Note:

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 27
Gametes, Fertilization and Parthenogenesis
> Fig. 25. Sperms of chordates, especially vertebrates. (Redrawn from E.B. Wilson)

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 22
Protoplasm and the cell
> Fig. 8. Scheme of spermatogenesis, oogenesis, fertilization and cleavage. Homologous stages of gamete development are on the same level.

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## Conservation and duplication of useful DNA

<figure><img src="figs/IMG_0221_fruitfly_mouse_hox_genes_0aaf428.jpg" width="400px"><figcaption>J. Allman, *Evolving Brains*</figcaption></figure>

Note:

Hox genes utilized in common ways between invertebrates and vertebrates

Evolving Brains.
p. 52.
> The sites of expression of the homeotic series of genes in the fruitfly embryo and the mouse embryo. The genes are expresed in the embryos in the same nos to tail arrangement as their order in the chromosomes. The homeotic series are replicated in four linear sets in the mouse chromosomes. The illusterted set (hox-2) is from chromosome 11.

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## Amphioxus

<figure><img src="figs/IMG_0172_amphioxus_a89525f.jpg" width="500px"><figcaption> *Fund of comp embyr vert*, A. Huettner 1958 p56</figcaption></figure>

Note:

amphioxus  
: lancelet  
: fish-like  
: benthic zone existence, live in subtidal sand flats in temperate and tropical locations, filter feeding on plankton from sea floor  
: Animalia => Chordata => => Cephalochordata

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 56
The embryology of amphioxus
> Fig. 28. Location of gonads in amphioxus.

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## Amphioxus embryo: fertilization

<figure><img src="figs/IMG_0173_amphioxus_embryo_fertilization_e9c6e08.jpg" width="500px"><figcaption>*Fund of comp embyr vert*, A. Huettner 1958 fig29</figcaption></figure>

Note:

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 57
Teh embryology of amphioxus
> Fig. 29. The egg of amphioxus at (a) fertilization, and (b) karyogamy. The black spheres represent yolk. The fertilization the firest polocyte has been formed and the second maturation division figure is in metaphase. Sperm drawn to scale at lower right in (b).

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## Amphioxus embryo: zygote cleavage

<figure><img src="figs/IMG_0174_amphioxus_embryo_zygote_cleavage_3861214.jpg" width="500px"><figcaption>*Fund of comp embyr vert*, A. Huettner 1958 fig31</figcaption></figure>

Note:

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 59
The embryology of amphioxus
> Fig. 31. The holoblastic cleavage of the amphioxus zygote as far as the 32-cell stage.

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## Amphioxus embryo: blastulation

<figure><img src="figs/IMG_0175_amphioxus_embryo_blastula_polarity_15f9a3b.jpg" width="400px"><figcaption>blastula of amphioxus, polarity change to gastrulation</figcaption></figure>

Note:

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 60
The embryology of amphioxus
> Fig. 32. The blasutla of amphioxus and its change in polarity during gastrulation. Teh change is cmpleted in Fig. 33. Compare this chagne i npolarity with that in the gasturla of the frog. Fig. 54.

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## Amphioxus embryo: gastrulation

<div><img src="figs/IMG_0176_amphioxus_embryo_gastrula_451e721.jpg" height="200"><figcaption>amphioxus gastrulation</figcaption></div>

<div><img src="figs/IMG_0177_amphioxus_embryo_gastrula_7bcc050.jpg" height="200"><figcaption>amphioxus gastrula, late</figcaption></div>

<div><img src="figs/IMG_0178_amphioxus_gastrula_neural_folds_417dadc.jpg" height="200"><figcaption>amphioxus, neural folds</figcaption></div>

Note:

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 61
The embryology of amphioxus
> Fig. 33. The gastrula of amphioxus with a remnant of the blastocoel still present at the blastopore.

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 63
The embryology of amphioxus
> Fig. 35. The gastrual of amphioxus. In the lower figure the ectoderm has grown over the blastopore in teh formation of the neurocoel. The passage between the neruocoel and the archenterion i s the neuroenteric canal.

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 64
The embryology of amphioxus
> Fig. 36. Gastrula of amphioxus showing progressive overgrowtth of the ectoderm over the blastopore in the formation of the neural folds

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## Amphioxus embryo: neurulation

Note:

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 65
The embryology of amphioxus
> Fig. 37. Amphioxus embryo with an archenteric or enterocoelic pouch in formation.

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 66
The embryology of amphioxus
> Fig. 38. Amphioxus embryo slightly older than that in preceding figure. The archenteric puch is constricted off and has formed a mesodermal somite.

Fundamentals of comparative embryology of the vertebrates. 1941, 1959, 1958 Alfred F. Huettner p. 67
The embryology of amphioxus
> Fig. 39. amphioxus embryo isth the neural tube, the notochord and the mesodermal somites differentiated.

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## Reference table of human development

<figure><img src="figs/IMG_0195_human_development_table_Arey1924_2df4a71.jpg" height="450px"><figcaption>Arey, Developmental Anatomy 1924, 1954</figcaption></figure>

Note:

Developmental Anatomy. Arey. 1924, 1954
p.107
A reference table of correlated human development

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## Reference table: human embryo size, mammal comp

<figure><img src="figs/IMG_0190_human_mammal_gestation_comparison_Arey1924_0b0e786.jpg" height="400px"><figcaption>human and mammal embryo dev tables, Arey 1924,1954</figcaption></figure>

Note:

Developmental Anatomy, 1924, 1954
p. 105
Relations of age, size, and weight in the human embryo.
Comparative data concerning gestation in mammals

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## Human embryo at three to four weeks

<figure><img src="figs/IMG_0186_human_embryo_day24_6e71b8a.jpg" width="500px"><figcaption>24d, 2.6mm, 19somites; 26d, 3.6mm, 25somites</figcaption></figure>

Note:

Developmental Anatomy 1924, 1954 Arey
p. 98
> Fig. 74 Human embryos of twenty-fout and twenty-six days, viewed from left side. A, Atwell embryo of 26 mm, with ninetween somites (Streeter; x23); B, 3.6mm, embryo with twenty-five somites (x16)

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## Human embryo at four to six weeks

<figure><img src="figs/IMG_0187_human_embryo_day35_fd26b22.jpg" width="500px"><figcaption>28d, 5mm; 35d, 8mm; 42d, 12mm; *Developmental Anatomy* 1954 Arey</figcaption></figure>

Note:

Developmental Anatomy 1924, 1954 Arey
p. 99
> Fig. 75 Human embryos of four and five weeks, viewed from the left side. A At 5 mm. B, at 8 mm
> Fig. 76. Human embryo of six weeks (12 mm). viewed from the right side.

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## Human embryo series

<figure><img src="figs/IMG_0189_human_embryo_series_Arey1924_8b70420.jpg" width="500px"><figcaption>*Developmental Anatomy* 1954 Arey fig78</figcaption></figure>

Note:

Developmental Anatomy 1924, 1954 Arey
p. 101
> Fig. 78. A graded series of human embryos, at natural size.

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## Human embryo limb development

<div style="margin-bottom:25px"><figcaption class="big">growth of human upper and lower limbs</figcaption><img src="figs/IMG_0196_human_embryo_limb_dev_6272398.jpg" width="500px"><figcaption>*Developmental Anatomy* 1954 Arey fig162</figcaption></div>

<div><figcaption class="big">sensory segments of upper limbs </figcaption><img src="figs/IMG_0197_human_limb_dev_forearm_innervation_segments_d0ec117.jpg" width="500px"><figcaption>*Developmental Anatomy* 1954 Arey fig 163</figcaption></div>

Note:

Developmental Anatomy 1924, 1954 Arey
p. 204
> Fig. 162. Stages in teh development of the human limbs between the figth and eight weeks. Upper row, upper limb; lower row, lower limb.

Developmental Anatomy 1924, 1954 Arey
p. 205
> Fig. 163. The segmental (sensory) innervation  (after Keegan and Garrett). A, At five weeks; B, in adult.

<!--
Human dermal hair groupings?

todo: need modern update to this information

Developmental Anatomy 1924, 1954 Arey
p. 444
> Fig. 400. The relation of hair groups to scales. A, Arrangement on the oppossums's tail (after Danforth and de Meijere). B, Arrangement inthe human fetus, with hypotehetical dermal scales drawn in (after Stohr).
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## Human eye development

<div><figcaption class="big">optic cup formation</figcaption><img src="figs/IMG_0207_human_optic_cup_retina_dev_962e0a1.jpg" width="400px"><figcaption>*Developmental Anatomy* 1954, Arey fig489</figcaption></div>

<div><figcaption class="big">early differentiation of human retinal cells</figcaption><img src="figs/IMG_0208_human_retina_dev_759f46d.jpg" width="400px"><figcaption>*Developmental Anatomy* 1954, Arey fig491</figcaption></div>

Note:

Developmental Anatomy 1924, 1954 Arey
p. 532
> Fig. 489. Human optic primordia, shown as models in side view. The lens is sections and the optic cup in A and C has been partly cut away. A, At 4.5 mm; B, at 5 mm; C, at 7.5 mm

Developmental Anatomy 1924, 1954 Arey
p. 534
> Fig. 491. EArly differentiation of the nervous layer of the human retina, shown in a vertical section at three months. At left Cajal's analysis of the component element after silver impregnation; at right the appearance with ordinary statins.
> Fig. 492. Later differentiation of the human retina. A, At seven months, in vertical section; at left, the cheif neurons shown by silver technique; at right, appearance with ordinary stains. B, Early cone cells during the figth month. C, rods and cones during the seventh month

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## Human eye development: malformations

<figure><figcaption class="big">cyclopia, adult persistence of pupillary membrane, colobomoa of iris</figcaption><img src="figs/IMG_0205_human_eye_dev_malformation_1b60991.jpg" width="500px"><figcaption>*Developmental Anatomy* 1954, Arey fig499</figcaption></figure>

Note:

Developmental Anatomy Arey, 1924, 1954
p. 542
> Fig. 499. Anomolies of the human eye. A, Cyclopia of a newborn, with a single eyeball but partial doubling of gthe lids; above the eye is the proboscis-like nose. B, Persistent pupillary membrane in an adult. C, Colobomoa of the iris.

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## Human embryo olfactory pit formation

<figure><img src="figs/IMG_0206_human_embryo_olfactory_dev_2c486d5.jpg" width="500px"><figcaption>*Developmental Anatomy* 1954, Arey fig479</figcaption></figure>

Note:

Developmental Anatomy 1924, 1954 Arey
p. 525
> Fig 479. Development of the human olfactory pits (after Peter). A, at 5 mm; B, at 11 mm
> Fig. 480. Development of the human olfactory pits. shown in transverse half-sections through the head. A, At 5 mm, at 7 mm, C, at 9 mm; D, at 12 mm

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## Human cerebral development

<figure><img src="figs/IMG_0204_human_embryo_cerebral_formation_a02a480.jpg" width="500px"><figcaption>*Developmental Anatomy* 1954, Arey fig458</figcaption></figure>

Note:

Developmental Anatomy Arey1924, 1954
p. 499
> Fig. 458. Invagination of the human choroid plexuses into the lateral and third ventricles, shown by a transverse section through a fetal brain of the third month.

> Fig. 459. Anomolies of the human brain. A, Micrencephaly, associated with a microcephalic head. B, Hydrocephalus, producing a macrocephalic head. C, Encephalocoele.

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## Human telencephalon formation

<figure><img src="figs/IMG_0201_human_embryo_telencephalon_coronal_fa70c49.jpg" width="400px"><figcaption>human telencephalon at 10mm</figcaption></figure>

Note:

Developmental Anatomy Arey1924, 1954
p. 490
> Fig. 448. Human telencephalon, at 10 mm. A, Left lateral view. B, Transverse section, at the level indicated on A (after Prentiss)

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## Human cerebrum at 2-3 months gestation

<figure><figcaption class="big">7 weeks gestation</figcaption><img src="figs/IMG_0199_human_brain_7weeks_432a090.jpg" width="400px"><figcaption>Arey fig444</figcaption></figure>

Note:

Developmental Anatomy Arey1924, 1954
p. 488
> Fig. 444. Human brain, in lefet lateral view (after Hochstetter). A, At 14 mm. B, at 27 mm
> Fig. 445. Human telencephalon and diencephalon, hemisected and viewed from the left side (after Hochstetter). A, At seven weeks; B, at ten weeks

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## Human cerebrum at 3-4 months gestation

<div><figcaption class="big">14 weeks gestation</figcaption><img src="figs/IMG_0203_human_embryo_cerebrum_14weeks_b1785c0.jpg" width="300px"><figcaption>Arey fig 454</figcaption></div>

Note:

Developmental Anatomy Arey1924, 1954
p. 495
> Fig. 454. Form and relations of the human cerebral hemisphere. A, Left lateral view of the brain, at fourteen weeks, in situ. B, Cast of the ventricular system in a newborn, viewed from above.

Developmental Anatomy Arey1924, 1954
p., 489
> Fig. 446. Human brain, hemisected, at fourteen weeks (after Hochstetter).
> Fig. 447. Stages in the development of the human epiphysis. A, Surface view at eight weeks (after Hochstetter;). B, Longituundinal hemisection, at ten weeks (after Hochstetter). C, Transverse section, at five months (after Gladstone)

> Fig. 454. Form and relations of the human cerebral hemisphere. A, Left lateral view of the brain, at fourteen weeks, in situ. B, Cast of the ventricular system in a newborn, viewed from above.

Developmental Anatomy Arey1924, 1954
p. 452
> Fig. 451. Human brain, at nearly three months, viewed from the right side after removal of most of the right pallium (His).
> Fig. 452. Human brain, at eighteen weeks, showing the rhinencephalon in ventral view (Hardesty after Retizius)

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## Human cerebrum at 7 months gestation

<div><figcaption class="big">Appearance of cortical fissures</figcaption><img src="figs/IMG_0210_human_brain_7months_3da29cb.jpg" width="500px"><figcaption>Arey fig 455</figcaption></div>

Note:

Gyri and sulci form. See the developmental mechanics behind this [^Tallinen:2006]

[^Tallinen:2006]: On the growth and form of cortical convolutions. http://dx.doi.org/10.1038/nphys3632

Developmental Anatomy Arey1924, 1954
p. 496
> Fig. 455. Right cerebral hemisphere, from a seven-months' fetus, in lateral view (Kollman)

* first fissures are large primary ones to appear at fourth month, the rhinal and hippocampal fissures. The lateral sylvian fissure also starts to appear but is not completed until after birth
 * at six to seventh months are when other major fissures appear such as teh central sulcus, the parieto-occiptal fissure, and the calcarine fissure

Developmental Anatomy Arey1924, 1954
p. 494
> Fig. 453. Human brain, at seven moths, hemisected (adapted from Kollman)