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Figure 5-1 Illustrations of folding of embryos during the fourth week. A1, Dorsal view of an embryo early in the fourth week. Three pairs of somites are visible. The
continuity of the intraembryonic coelom and extraembryonic coelom is illustrated on the right side by removal of a part of the embryonic ectoderm and mesoderm. B1, C1,
and D1, Lateral views of embryos at 22, 26, and 28 days, respectively. A2 to D2, Sagittal sections at the plane shown in A1. A3 to D3, Transverse sections at the levels
indicated in A1 to D1.
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Figure 5-2 Folding of cranial end of embryo. A, Dorsal view of embryo at 21 days. B, Sagittal section of cranial part of the embryo at the plane shown in A. Observe the
ventral movement of the heart. C, Sagittal section of an embryo at 26 days. Note that the septum transversum, primordial heart, pericardial coelom, and oropharyngeal
membrane have moved onto the ventral surface of the embryo. Observe also that part of the umbilical vesicle is incorporated into the embryo as the foregut.
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Figure 5-3 Illustrations of the effect of the head fold on the intraembryonic coelom. A, Lateral view of an embryo (24 to 25 days) during folding, showing the large
forebrain, ventral position of the heart, and communication between the intraembryonic and extraembryonic parts of the coelom. B, Schematic drawing of an embryo (26
to 27 days) after folding, showing the pericardial cavity ventrally, the pericardioperitoneal canals running dorsally on each side of the foregut, and the intraembryonic
coelom in communication with the extraembryonic coelom.
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Figure 5-4 Folding of caudal end of the embryo. A, Lateral view of a 4-week embryo. B, Sagittal section of caudal part of the embryo at the beginning of the fourth week.
C, Similar section at the end of the fourth week. Note that part of the umbilical vesicle is incorporated into the embryo as the hindgut and that the terminal part of the
hindgut has dilated to form the cloaca. Observe also the change in position of the primitive streak, allantois, cloacal membrane, and connecting stalk.
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Figure 5-5 Schematic drawing of derivatives of the three germ layers: ectoderm, endoderm, and mesoderm. Cells from these layers contribute to the formation of different
tissues and organs, e.g., the endoderm forms the epithelial lining of the gastrointestinal tract and the mesoderm gives rise to connective tissues and muscles.
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Figure 5-7 Sketches of three possible methods of transmission of signal substances in inductive cell interactions. A, Diffusion of signal substances. The signal appears to
take the form of a diffusible molecule that passes from the inductor to the reacting tissue. B, Matrix-mediated interaction. The signal is mediated through a nondiffusible
extracellular matrix, secreted by the inductor, with which the reacting tissue comes in contact. C, Cell contact-mediated interaction. The signal requires physical contact
between the inducing and responding tissues. (Modified from Grobstein C: Adv Cancer Res 4:187, 1956; and Saxen L: In Tarin D [ed]: Tissue Interactions in
Carcinogenesis. London, Academic Press, 1972.)
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Figure 5-8 A and B, Drawings of dorsal views of embryos early in the fourth week showing 8 and 12 pairs of somites, respectively. C, D, and E, Lateral views of older
embryos showing 16, 27, and 33 pairs of somites, respectively. The rostral neuropore is normally closed by 25 to 26 days, and the caudal neuropore is usually closed by
the end of the fourth week.
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Figure 5-9 A, Dorsal view of a five-somite embryo at Carnegie stage 10, approximately 22 days. Observe the neural folds and deep neural groove. The neural folds in the
cranial region have thickened to form the primordium of the brain. B, Illustration of the structures shown in A. Most of the amniotic and chorionic sacs have been cut away
to expose the embryo. C, Dorsal view of an older eight-somite embryo at Carnegie stage 10. The neural tube is in open communication with the amniotic cavity at the
cranial and caudal ends through the rostral and caudal neuropores, respectively. D, Diagram of the structures shown in C. The neural folds have fused opposite the
somites to form the neural tube (primordium of spinal cord in this region). (A and C, From Moore KL, Persaud TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed.
Philadelphia, WB Saunders, 2000.)
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Figure 5-13 A, Drawing of an embryo at Carnegie stage 13, approximately 28 days. B, Photomicrograph of a section of the embryo at the level shown in A. Observe the
hindbrain and otic vesicle (primordium of internal ear). C, Drawing of same embryo showing the level of the section in D. Observe the primordial pharynx and pharyngeal
arches. (B and D, From Moore KL, Persaud TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed. Philadelphia, WB Saunders, 2000.)
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Figure 5-16 A, Lateral view of an embryo at Carnegie stage 14, approximately 32 days. The second pharyngeal arch has overgrown the third arch, forming a depression
known as the cervical sinus. The mesonephric ridge indicates the site of the mesonephric kidney, an interim kidney (see Chapter 12). B, Illustration of the structures
shown in A. The upper limb buds are paddle shaped and the lower limb buds are flipper-like. (A, From Nishimura H, Semba R, Tanimura T, Tanaka O: Prenatal
Development of the Human with Special Reference to Craniofacial Structures: An Atlas. Washington, DC, National Institutes of Health, 1977.)
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Figure 5-17 A, Lateral view of an embryo at Carnegie stage 17, approximately 42 days. Digital rays are visible in the handplate, indicating the future site of the digits. B,
Drawing illustrating the structures shown in A. The eye, auricular hillocks, and external acoustic meatus are now obvious. (A, From Moore KL, Persaud TVN, Shiota K:
Color Atlas of Clinical Embryology, 2nd ed. Philadelphia, WB Saunders, 2000.)
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Figure 5-18 A, Lateral view of an embryo at Carnegie stage 19, about 48 days. The auricle and external acoustic meatus are now clearly visible. Note the relatively low
position of the ear at this stage. Digital rays are now visible in the footplate. The prominence of the abdomen is caused mainly by the large size of the liver. B, Drawing
indicating the structures shown in A. Observe the large hand and the notches between the digital rays, which clearly indicate the developing digits or fingers. (A, From
Moore KL, Persaud TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed. Philadelphia, WB Saunders, 2000.)
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Figure 5-19 A, Lateral view of an embryo at Carnegie stage 21, approximately 52 days. Note that the feet are fan shaped. The scalp vascular plexus now forms a
characteristic band across the head. The nose is stubby and the eye is heavily pigmented. B, Illustration of the structures shown in A. The fingers are separated and the
toes are beginning to separate. C, A Carnegie stage 20 human embryo, approximately 50 days after ovulation, imaged with optical microscopy (left) and magnetic
resonance microscopy (right). The three-dimensional data set from magnetic resonance microscopy has been edited to reveal anatomic detail from a mid-sagittal plane.
(A, From Nishimura H, Semba R, Tanimura T, Tanaka O: Prenatal Development of the Human with Special Reference to Craniofacial Structures: An Atlas. Washington,
DC, National Institutes of Health, 1977; B, From Moore KL, Persaud TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed. Philadelphia, WB Saunders 2000; C,
Courtesy of Dr. Bradley R. Smith, Center for In Vivo Microscopy, Duke University Medical Center, Durham, NC.)
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Figure 5-20 A, Lateral view of an embryo at Carnegie stage 23, approximately 56 days. The embryo has a distinct human appearance. B, Illustration of the structures
shown in A. C, A Carnegie stage 23 embryo, approximately 56 days after ovulation, imaged with optical microscopy (left) and magnetic resonance microscopy (right). (A,
From Nishimura H, Semba R, Tanimura T, Tanaka O: Prenatal Development of the Human with Special Reference to Craniofacial Structures: An Atlas. Washington, DC,
National Institutes of Health, 1977; B, From Moore KL, Persaud TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed. Philadelphia, WB Saunders, 2000; C,
Courtesy of Dr. Bradley R. Smith, Center for In Vivo Microscopy, Duke University Medical Center, Durham, NC.)
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Figure 5-21 Lateral view of an embryo and its chorionic sac at Carnegie stage 23, approximately 56 days. Observe the human appearance of the embryo. (From
Nishimura H, Semba R, Tanimura T, Tanaka O: Prenatal Development of the Human with Special Reference to Craniofacial Structures: An Atlas. Washington, DC,
National Institutes of Health, 1977.)
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Figure 5-23 Illustrations of methods used to measure the length of embryos. A, Greatest length (GL). B and C, Crown (C)-rump (R) length. D, Crown (C)-heel (H) length.
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